A former World War II bunker in the St. Pauli area of Hamburg, Germany, is being transformed, in a $63-million project with five additional floors of new, vegetation-covered cultural and sports spaces and a modern hotel.

The Flak Tower IV was built with slave labor around 1942. It sheltered up to 25,000 people at a time under its 5-m-thick concrete roof during Allied air raids.

Since the 1990s, under the control of developer Matzen Immobilien, the 40-m-tall concrete block with a square footprint of 75 m per side has had various commercial and cultural uses, including a nightclub, music store and music school.

During work that started in 2019, Matzen is adding five tapering floors supported by 16 large concrete-filled steel columns atop the bunker's walls, which are up to 4.5 m thick.

Steelwork brackets, weighing around 5.5 metric tons each, are being attached to the bunker walls to support a 300-m-long, 5-m-wide "mountain path" wrapping around the building from ground level to the roof garden.

Nearly 5,000 trees, shrubs, hedges and climbing plants will enhance the project, which is due for completion this year, according to a Matzen spokesman.