Since 1975, the Rotterdam-based Office for Metropolitan Architecture has produced some of the world's most provocative buildings. Led by Rem Koolhaas and his nine partners, the firm's most notable built projects include seminal works such as the CCTV Headquarters in Beijing, the Seattle Central Library, and Casa da Musica in Porto, Portugal. Known as one of the world's leading creators of boundary-pushing design, OMA's influence on the global architectural landscape is undeniable.

Among the firm's several hundred realized projects, however, many lesser known proposals were drafted but never constructed. Arguably a fundamental component of the OMA's practice, the unbuilt projects contain some of the firm's most outlandish and important ideas with incredible potential to influence architectural design worldwide. As a tribute to Koolhaas and OMA's continued pursuit of the unconventional, we've rounded up fifteen of OMA's most unusual unbuilt skyscrapers. Read on to find out which ones made the list.

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Hyperbuilding

Perhaps OMA's most eccentric unbuilt tower, Hyperbuilding was proposed in 1996 as a fusion of buildings into a series of wild interconnected extrusions on the banks of the Chao Phraya river in Bangkok, Thailand. Designed to house approximately 120,000 people, Hyperbuilding was envisioned as a self-contained city with its own comprehensive internal transportation system. According to the project brief from OMA, the building would have included a series of cable cars, gondolas, train elevators, high and low speed elevators and a walkable twelve kilometer promenade. The ambitious program for the one-kilometre tall Hyperbuilding included housing, office, education, public space and more.

The project was a commissioned study, never advancing beyond concept development. According to BD Online, the project was revived ten years later in Louisville, Kentucky, but would remain unbuilt despite a scaled-down design.

Idea Vertical Campus

Save this picture! IDEA Vertical Campus. Image Courtesy of OMA

Set in Tokyo's rapidly expanding Shinjuku ward, Idea Vertical Campus was designed to disrupt the monotonous landscape of towers that have come to define the architecture of the city. Recently becoming known for his vehement critiques of the contemporary architecture of cities, Koolhaas deems linear glass towers devoid of personality - so it should come as no surprise that Koolhaas envisioned a pixelated design for the Idea Vertical Campus, visualizing the tower as a fluid entity allowing distinct identities for the three schools inside. Conceived as visual reorganization of the typical skyscraper, Idea Vertical Campus questioned the standardization of urban landscapes and the influence of skyscraper geometry on spatial programming.

Idea Vertical Campus was designed for a competition in 2004 that had one mandate for architects: no rectangular towers would be permitted. Japanese architect Kenzo Tange was selected as the competition winner with Mode Gakuen Cocoon Tower, an ellipse-shaped "cocoon" inspired structure.

MoMA Charette

Save this picture! MoMA Charette. Image Courtesy of OMA

In 1997, The Museum of Modern Art held a competition for a new building with a particularly challenging but typically New York-sized plot: the museum was sandwiched between a series of towers, allowing for little room to build. Koolhaas proposed a simultaneous upward-downward expansion that incorporated a pyramid-like tower above ground connected to a catacomb of gallery spaces below. Koolhaas felt that the notion of the museum was "at the brink of a quantum leap," requiring an entirely new typology for its design. Based on a self-imposed desire for "newness," OMA produced a collage of spaces optimized for a technology-rich, yet unpredictable future.

Koolhaas' proposed museum-tower-hybrid was ultimately bested by Japanese architect Yoshio Taniguchi who proposed a series of rectilinear gallery spaces to fill the programmatic void. MoMA's Taniguchi-designed Manhattan location opened in 2004.

Koningin Julianaplein

Save this picture! Koningin Julianaplein. Image Courtesy of OMA

The city of The Hague in The Netherlands has long been a focus of OMA's practice. The firm has a storied history with The Hague that begins with one of the firm's earliest projects, a proposal in 1978 for an extension to the Dutch Parliament. Located between the firm's native Rotterdam and Amsterdam, The Hague has been in a development boom for decades led by an influx of international business and political ventures.

In 2002, a series of plots along Koningin Julianaplein and adjacent to The Hague Central Station were identified by civic government as a potential area for development. In 2002, OMA won a competition for a residential development tower spanning several plots, extending above motorways and green space. Designed as a bridge to connect the city's business hub with nearby residential areas, the eccentric proposal included 179 apartments staggered across a series of linked towers.

In 2010, The Hague government voted in favour of proceeding with the project despite eight years of debate. Now thirteen years after the winning proposal was chosen, the project remains in limbo.

Dubai Renaissance

Save this picture! Dubai Renaissance. Image Courtesy of OMA

Rising from the sandy shores of the Emirate of Dubai, a monolith of unbelievable proportions sits staunchly in front of all the other towers, equivalent to the height of the Eiffel Tower. Add a rotating foundation, throw in housing, offices, hotels, public space, parking and retail, and you have Rem Koolhaas' perfect recipe for a self-contained city. Meet Dubai Renaissance, the all-encompassing tower designed to preside over the city's central harbour from every angle. Inspired by a desire to challenge the notion of iconic architecture, Koolhaas sought to construct a single volume that "wastes no energy on useless invention."

The "anti-iconic" tower was proposed in a 2006 contest for a new tower in the heart of Dubai, ultimately won by Zaha Hadid's futuristic "Signature Towers." Despite the loss OMA was rumoured to have continued work on the project in a new location on the Emirate, albeit without its rotating foundation.

UN City

Save this picture! UN City. Image Courtesy of OMA

Self-contained cities have pervaded OMA's proposals for skyscrapers, led by Koolhaas' vision for a deeply integrated urban environment built on principles of coexistence. UN City, proposed for New York in 2001, was designed as a collection of interconnected slender towers that diverged from the typical rectangular Manhattan skyscraper in favour of jagged edges and asymmetry. Linked by a series of suspended pedestrian ramps, UN City incorporated urban agriculture, housing and commercial space, among other functions. Each tower different from the next, OMA imagined a sprawling village of vertical cohabitation in America's most densely populated city. "We imagine a new kind of towers and eschew the programmatic stagnation that has rigidly shaped New York," says the project brief, "They are towers for 21st century living."

UN City was proposed for a masterplan competition in 2001 on property adjacent to the project's namesake, the United Nations.

Torre Bicentenario

Save this picture! Torre Bicentenario. Image Courtesy of OMA

Created to celebrate the 200th anniversary of Mexico's War of Independence, Torre Bicentenario was envisioned as a monument to post-war success in a rapidly developing country. The soaring tower would have been the tallest in Latin America with a design that featured a 22-story open air atrium punctured halfway up the building, at the design's widest point. Comprising two enormous pyramids connected by a garden, the angular structure was designed to form a stark contrast against Mexico City's endless landscape of three-story structures. Another in OMA's series of mixed-use towers, Torre Bicentenario was programmed to include a host of government services, office space, retail and an auditorium.

Originally slated to open in September 2010, the project was mired in controversy and backlash from the beginning over zoning regulations that forbade skyscrapers, further enhanced by local residents vehemently opposed to its construction. Funding from the government of Mexico City became unsteady as the project lost public support, consequently leading to its cancellation in 2007.

La Defense Projet Phare

Save this picture! La Defense Projet Phare. Image Courtesy of OMA

Proposed for Paris' expanding business district La Defense, Projet Phare would shatter preconceived notions of skyscraper design. Known in the city as a booming district of commerce defined by its modern glass skyline, the towers of La Defense rise in contrast to classical Parisian architecture. La Defense is home to an estimated 37.7 million square feet of office space across 18 skyscrapers. OMA crafted a tower that would celebrate the modernity of La Defense while disrupting its monotonous skyscraper landscape. The tower is defined by four rectangular volumes that jut abruptly outwards from the tower's mesh façade, containing a series of restaurants with some of the best views Paris has to offer.

Designed for a 2006 competition for La Defense, OMA was ultimately bested by Los Angeles-based Morphosis Architects led by Thom Mayne. Construction is now underway with completion slated for 2017.

C3 Maastowers

Save this picture! C3 Maastowers. Image Courtesy of OMA

Before the construction of UN Studio's renowned Erasmus Bridge that now links formerly disparate communities along the Maas river, Rotterdam struggled with solutions to connect the north and south of the city. C3 Maastowers was envisioned as the solution: rather than building another bridge, Koolhaas proposed the construction of a series of towers unified by a central volume to create a self-contained city. Located in a formerly industrial quarter, the towers were designed to convince residents of the viability of living on Rotterdam's uninhabited south bank. The towers were slated to include a cinema, an athletic center, shops, housing and offices.

According to OMA, "our proposal was complex, would have been expensive and had to compete with other projects in the center," ultimately leading to its demise in 1994. OMA's proposal inspired the City of Rotterdam to begin development in the area, subsequently launching a search for ideas by the city's architects. Slightly over a decade later, OMA unveiled De Rotterdam, a mixed-use tower on the same plot. Arguably similar to C3 Maastowers, De Rotterdam is the largest building in The Netherlands.

425 Park Avenue

Save this picture! 425 Park Avenue. Image Courtesy of OMA

In 2012, OMA began its most recent proposal for a Manhattan skyscraper. Challenged by a client who sought the ideas of eleven of the world's most prolific firms including Zaha Hadid Architects, Rogers Stirk Harbour + Partners, and Foster + Partners, OMA produced a tower unlike any other in the New York skyline. 425 Park Avenue soars from the streets of Manhattan and shatters the linear tendencies of its modernist neighbors, opting for a contemporary asymmetrical volume clad entirely in glass.

425 Park Avenue was designed by OMA partner Shohei Shigematsu and included the preservation of 25% of the existing building. The competition was ultimately won by Foster + Partners whose linear design is said to emulate that of the nearby Seagram Building and Lever House.

23 East 22nd Street

Save this picture! 23 East 22nd Street. Image © Luxigon

Travel two dozen blocks south of 425 Park Avenue and you'll arrive at the site of another proposed and yet-unbuilt OMA skyscraper, 23 East 22nd Street. Positioned on a plot measuring just 33 feet (10.5 meters) wide, the tower's footprint extends far beyond its ground-level width, cantilevering 30 feet over the neighboring building. The 24-story tower belongs to the final phase of a major redevelopment of One Madison Square Park by Slazer Enterprises, a project anchored by a 50-story tower inhabited by the likes of Rupert Murdoch and Gisele Bündchen. The provocative tower was said to include a screening room for the Creative Artists Agency along with 18 luxury private residences.

Since its announcement in 2007 at the beginning of the American recession, the tower has been caught in financial and governmental red tape. Although the project remains officially in design development, permits issued by the city allowed only for a six-storey tower, negating OMA's design. Slazer Enterprises has since completed construction on the six-story building at 23 East 22nd Street, serving as the entrance to the adjacent tower at One Madison Square.

India Tower

Save this picture! India Tower. Image Courtesy of OMA

"Mumbai lacks an architectural symbol that projects its cosmopolitan identity," writes OMA. "The imminent arrival of towers emulating the Dubai formula appears equally dubious as a means for Mumbai to express itself." Proposed in 2008 as Mumbai's first iconic skyscraper, Koolhaas envisioned India Tower as an emblem for the future of architecture in the developing nation. Poised to redefine the city's largely six-story skyline, OMA's design created the illusion of a cylindrical tapered tower topped by an identical inverted volume, connected only by a narrow mirrored cylinder at the center. The Park Hyatt Tower, as it was officially known, would have housed the Mumbai flagship location of the international hotelier along with a series of private residences, a theater, retail, and a prominent viewing platform situated between the inverted volumes.

Plans for the tower were abandoned almost immediately after the 2008 competition announcement due to the financial crisis. In 2010, developer Dynamic Balwas Group announced plans for an 85-story supertall tower at the same location designed by Foster + Partners. Construction on the tower was halted by a stop-work order issued by the City of Mumbai in 2011 and has yet to resume.

Scotts Tower

Save this picture! Scotts Tower. Image Courtesy of OMA

Located adjacent to the luxurious Orchard Road shopping district, Scotts Tower was envisioned to become Singapore's newest luxury address. Composed of four autonomous towers connected through a central volume, the building was set to include 68 high-end residences ranging from single and two-story apartments to a penthouse with a rooftop terrace. Designed in true OMA fashion, the residential volumes were staggered along the central axis to juxtapose the staid architecture of nearby towers.

The project was led by then-OMA partner Ole Scheeren, undergoing concept and design development throughout 2006 and 2007. In December 2011, Amsterdam-based UN Studio announced their design for Scotts Tower initiated by developer Far East Organization, the originators of OMA's commission for the same location. The 31-storey tower will be home to 231 upscale apartments with a smaller footprint than OMA's original design. The tower is currently under construction with opening slated for 2020.

The Twins

The city of Tunis, Tunisia lies at Africa's northernmost point, just south of the Italian city of Palermo. First settled during the 6th Century BC, the ancient city is home to a plethora of monuments celebrating the region's storied history. Now home to 2.7 million residents, Tunis is in the midst of an urban revolution as glass towers gradually appear on a skyline. In 2008, a competition was held by UAE-based Sama Dubai for the development of a major waterfront property just south of the Lake of Tunis. Serving as the anchor of the development, OMA proposed "The Twins": two identical towers slated to include residential accommodation, a hotel, office space and retail.

Led by partners Rem Koolhaas and Reinier de Graaf, the project remains OMA's only foray into Tunisia. With masterplan costs estimated at $25 billion USD in 2007, the project was projected to be one of the biggest in Tunisia's history with "The Twins" serving as the anchor to the development. The project became embroiled in scandal after facing severe scrutiny for its size and cost and has consequently been placed on hold indefinitely by the developer.

111 First Street

Save this picture! 111 First Street. Image Courtesy of OMA

Located across from Lower Manhattan on the Hudson River, 111 First Street was designed to activate Jersey City's fast developing skyline. Although designed to harmonize with the city's landscape, OMA's 52-story tower departs from adjacent tower typologies through the creation of four distinct rectangular volumes. According to OMA, "The stacking maintains the independence of each block, optimizes views from the site and creates a dynamic relationship between the building and its surroundings: spectacle from convention." Each of the tower's four autonomous blocks contains a different program: residential, hotel, artist studios and residences, and gallery space, punctuated by outdoor terraces at each junction. Similar to De Rotterdam, programmatic delineations became the guiding design strategy for the tower, asserting its individuality in a city defined by a monotony of towers.

Perhaps the likeliest to be realized of all of OMA's unbuilt projects, 111 First Street was approved prior to the recession with no apparent roadblocks to its construction. Similar to OMA's projects from the same period, 111 First Street was delayed during the recession and has yet to break ground on its lucrative waterfront site. Nine years since inception, the project remains in financial limbo despite having cleared all governmental hurdles.

Did your favourite unbuilt OMA tower make the list? Let us know in the comments below.