Art patrons Christian and Karen Boros have thrown open the doors this week to their own personal World War II air raid bunker in Berlin, showing off gems from their 700-work collection. A tree made of found objects by Chinese dissident artist Ai Weiwei, Turner prize winner Wolfgang Tillmans's edgy photography and giant spider-web installations by Tomas Saraceno all resonate against a one-of-a-kind backdrop. In the centre of the German capital, near the old Cold War border crossing at Friedrichstrasse station, the vast and gloomy concrete above-ground cube has over the last four years become one of the city's top artistic attractions. In May 2008, a first selection of works from the couple's private collection went on display, attracting around 120,000 visitors until it closed earlier this year for a top-to-bottom overhaul. "It was a difficult decision to take" to close the original exhibition because most artists featured in it had personally installed their creations themselves, Karen Boros said at a preview of the show. "But we said to ourselves that if we were going to change something we should change everything." Karen Boros and her Polish-born husband, who made his fortune in advertising in western Germany, delved into their archive to decide which works should now get a turn in the spotlight. All the artists now featured have ties to Berlin, where many air raid shelters and disused industrial spaces have won a new lease of life as cultural venues. "The oldest work dates from 1990 and the most recent, six hours ago," Christian Boros said last week. "Thomas Zipp, who has a key to the bunker, came to install his work overnight," said the collector, who takes pride in the relationships he has developed with the artists he supports. "It's not as if the artists don't care how their works are shown," said Boros, especially "in an exhibition space that is far from optimal like this bunker". But the historically evocative space lends a powerful aura to the works. Built in 1942 according to plans by Albert Speer, Hitler's favourite architect, the bunker was designed to protect 2,500 people from aerial bombing during World War II. The Soviet occupiers used it as a prison for two years after the war, and it became the "Banana Bunker" in communist East Germany as a storage site for tropical fruits sent from brother country Cuba. After the Berlin Wall's fall, it became one of the most popular techno clubs of the capital and later notorious for its decadent sex parties -- traces of fluorescent paint on the walls recall the bunker's former incarnation. It was probably "the noisiest nightclub in the world", said Christian Boros, noting the thick concrete walls, absence of windows and low ceilings allowing sound to resonate in the smoky rooms. The couple bought the five-storey structure in 2003 and perched a spectacular, sunlight-flooded apartment on the roof. Christian Boros admitted that deciding on works was not always easy for spouses with extremely distinct tastes. "When you collect art as a couple, there are really three people collecting -- me, my wife and the two of us together," he joked. He pointed to the installation "Teenage Room" by Sweden's Klara Liden, an elaborate black bunk-bed creation built for the Venice Biennale in 2009. "My wife adores it but I can't stand it," he admitted with a smile.