The numbers don’t lie. At New York’s Museum of Modern Art, 24 of 1,221 works by Pablo Picasso in the institution’s permanent collection can currently be seen by visitors. Just one of California conceptual artist Ed Ruscha’s 145 pieces is on view. Surrealist Joan Miró? Nine out of 156 works.

The walls of the Tate, the Met, the Louvre or MoMA may look perfectly well-hung, but the vast majority of art belonging to the world’s top art institutions (and in many countries, their taxpayers) is at any time hidden from public view in temperature-controlled, darkened, and meticulously organised storage facilities. Overall percentages paint an even more dramatic picture: the Tate shows about 20% of its permanent collection. The Louvre shows 8%, the Guggenheim a lowly 3% and the Berlinische Galerie – a Berlin museum whose mandate is to show, preserve and collect art made in the city – 2% of its holdings. These include approximately 6,000 sculptures and paintings, 80,000 photographs, and 15,000 prints by artists including George Grosz and Hannah Höch.

“We don’t have the space to show more,” says Berlinische Galerie director Thomas Köhler, explaining that the museum has 1,200 sq m in which to display works acquired over decades through purchases and donations. “A museum stores memory, or culture,” explains Köhler. But here, like in other museums around the world, many works rarely if ever see the light of day.

A spatial deficit is only one reason why not. Another is fashion: some holdings no longer fit their institutions’ curatorial missions. Lesser works by well-known artists may also languish – their hits hang on museum walls; their misses lie forgotten in flat files. Works that come to a museum within estate acquisitions “might sit around in crates for years, waiting to be sorted,” explains Köhler. Some works stay under wraps due to delicacy or damage – and different institutions have varied storage and rotation policies, depending on a collection’s nature and scope. While London’s National Gallery uses a double hang system, thereby increasing the number of its permanent works on view, the Albertina in Vienna possesses more than a million Old Master prints – many of them centuries old and very sensitive. The percentage on view is thus very low, even if most of the holdings are kept onsite. (Other museums keep their caches in secret offsite warehouses.)

“Having 5% of your national collection on show is something people find difficult to understand,” says British curator Jasper Sharp, who was the commissioner of the Austrian pavilion at the 2013 Venice Bienniale. Many art institutions are thus coming up with ways to show their stuff, so to speak. “There is a great move to open up collections,” adds Sharp. Besides digitising images of the permanent collection (which many major institutions are currently in the process of doing), one way to display holdings is the idea of the Schaulager (translation: ‘storage display’) – in which visitors can see works archived, on sliding racks, behind glass, or during restoration. The Hermitage’s storage facility opened in 2014 and offers guided tours of collections long unseen; a number of US museums, like the Brooklyn Museum of Art have also created accessible storage centres. Other museum expansions – the Tate, the MoMA, and the Met are just a few currently underway – are meant to increase space for permanent collection viewing.

Until visible storage is everywhere – or museums grow so large that everything is on view, like a massive database – here are a few examples of wonderful things not often seen, and why.

Albrecht Dürer, Young Hare (1502)

Albertina Museum, Vienna