For five days every year, the population of Saudi Arabia increases by 3 million. That’s when Muslims visit Mecca for one of the religion’s most sacred rites: the hajj. Most cities aren’t built with that kind of surge capacity. And as the tragic stampede at this year's gathering showed, at a certain density those crowds can become dangerous. So the Saudis have, over the years, turned to a series of the world’s best architects and designers to try to keep millions of pilgrims safe, healthy, awed, and (a few of them) very, very comfortable—while honoring the tenets of Islam. The result is a city of carefully regulated experiences, with more work yet to be done. —Tim De Chant

A Purpose-Built Airport for a Massive Wave of Visitors

Bryan Christie Design

Inspired by traditional Bedouin tents, the 2.8-million-square-foot Hajj Terminal at King Abdulaziz International Airport has two massive cable-stayed, Teflon-coated fiberglass roofs. Passengers grab their luggage in the climate-controlled section; natural ventilation in the open-air waiting area keeps temperatures around 80 degrees Fahrenheit, even while they hit a searing 120 outside. There pilgrims wait—sometimes up to 36 hours—for their ride to Mecca, 43 miles to the east. —Tim De Chant

Disappear 600 Tons of Trash a Day With Pneumatic Tubes

The $20 billion-plus the Saudi government has spent on mosque improvements will solve at least one unholy problem: 600 tons of trash a day. It’ll disappear into 400 openings and get sucked at 40 mph through an underground network of pneumatic tubes to a station more than a mile away, where trucks will take it to a landfill. —Juliette Spertus

Brown Bird Design### Expanding the Great Mosque to Handle Bigger Crowds

Bryan Christie Design

The Masjid al-Haram, the Great Mosque of Mecca, is an ever-expanding house of worship built around the holiest shrine in all of Islam—the cloth-draped granite cube known as the Kaaba. Pilgrims walk seven times counterclockwise around the Kaaba twice on their hajj trip, and in recent years crowds have been so thick that they’ve forced people onto the mosque’s roof. Architects from the firm Gensler took a cue from those rooftop pilgrims and proposed a series of eight-sided platforms surrounding the Kaaba. Upgrading the mosque has been a delicate balancing act, says Bill Hooper, head of Gensler’s transportation practice. “We needed to keep them as close in to the Kaaba as we could but not compromise the emotional experience.” The solution, Hooper says, will improve traffic flow, double capacity, and maintain sight lines. Buildings around the Masjid al-Haram have been received less fondly, though. Abraj Al-Bait is the least-likely second-tallest building in the world, dominated by a Big Ben-like central clock tower that looms 1,972 feet above the mosque. The hotel is augmented by a light show at night. —Tim De Chant

A Hotel for the 0.01 Percent

Planned for completion in 2017, the Abraj Kudai will be the biggest hotel in the world—12 towers, 10,000 hotel rooms, 70 restaurants, four helipads, a shopping mall, and a bus station. Five floors are said to be reserved for the exclusive use of the Saudi royal family; the merely wealthy will have to make do with standard accommodations. It’s a decidedly odd, neoclassic addition to the site. —Tim De Chant

Brown Bird Design### Erect a City of 100,000 Fiberglass Tents

Most pilgrims spend nights in the Mina Valley—the hajj’s midpoint—in a vast tent city. For decades the sandy plain was crammed with simple cotton tents, but a deadly fire in 1997 sent the Saudi government searching for a durable fireproof replacement. Now an extensive network of permanent hydrants helps protect more than 100,000 air-conditioned, semipermanent structures of Teflon-coated fiberglass, each ranging from 250 to 850 square feet. And since close quarters can also aid the spread of illness, Mina has dedicated hospitals and ambulances too. Safety is still an issue here, though—it was in Mina that a stampede killed at least 700 people during this year's Hajj. —Tim De Chant

Bryan Christie Design

Adding Levels to a Bridge Where Pilgrims Throw Stones at the Devil

Pilgrims come to the Jamarat Bridge on the third day of the hajj for the Stoning of the Devil—to throw seven pebbles at each of the three jamarat, pillars representing the three times Abraham refused the devil’s challenges. The bridge was the site of several stampedes in the '00s, one of the most dangerous parts of the crowded pilgrimage, before architects built the area around the jamarat into a long five-level bridge. Now people have better access to the pillars, and a handful of exit ramps can now help quickly clear the structure, though the area still gets crammed with people. —Tim De Chant

Joe Mckendry

Mecca is just one of eight cities that show you what the future will look like, part of our 2015 Design Issue. You can see the rest here, exclusively on WIRED.com, and on newsstands now.