Proposals to revamp the bunker came thick and fast in the 21st century. Leaseholder Thomas Matzen rejected concepts ranging from a glass enclosure enveloping the entire structure to a 223-feet (68m) sphere on the roof and additional office floors, according to the Hamburger Abendblatt newspaper.

Then in October 2013, entrepreneur Mathias Muller-Using and local resident Sonja Brier came up with the idea of greening the roof to create a community sky garden. It would be accessed via a metal green 'mountain path' inspired by New York's High Line (pictured), the popular elevated park built on an old railroad track; and the Niteroi Contemporary Art Museum's ramp in Rio de Janeiro.

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The remarkable reinvention of this massive Nazi fortress in Hamburg