It’s a quarter past nine in the evening when Christian Boros slides open the large wooden door that separates the penthouse he shares with his wife, Karen, and 8-year-old son, Anton, from the five floors of exhibition space below. The residence is light and airy, clad in glass in the mode of Berlin’s recent architecture, but the private museum occupies a heavy concrete structure that was built as a bunker by the Nazis. It served as a storage facility for tropical fruit for the elite when the section of the city where it stands was under East German control, then became home to a techno club after reunification. Boros opened the 30,000-square-foot, 80-room exhibition space in 2008.

“Sorry I’m late; Wolfgang just finished installing the last photo,” he says. Christian is referring to Wolfgang Tillmans, who 
was on hand to assist with the installation of 38 of his own works—some of which picture the building in its party days—
as part of a new presentation of the couple’s formidable collection of contemporary 
art that is to be unveiled in early fall to a gathering of the artists and a few close friends.

Comprising 101 pieces from their current holdings of roughly 700 works, it is only the second installation at the bunker. The rehanging had been expected to come much sooner. “After a year or two, we wanted to change the exhibition because we are so proud to have wonderful works,” says Christian. “But then we thought, why? It’s not about showing power or how big our collection is.”

Even after four years displaying the first exhibition, which featured 110 pieces and was seen by more than 120,000 visitors, Karen says there was a tremendous public outcry via e-mails and phone calls when they announced it would come down. The couple acknowledge many more would have been able to visit the space if not for their strict appointment-only policy, necessary because of the 12-person occupancy limit placed on the building. The limit was imposed because they didn’t want to disrupt the rooms with exit signs.

One reason the first exhibition stayed up for four years was that many of the works required intricate planning and construction to show in the first place. “People told us we were crazy to remove some pieces,” says Christian. “Works like Monika Sosnowska’s black lightning bolt [Untitled, 2005/08] and Santiago Sierra’s Layered Tar Forms [2002] were so difficult to install. It took one month just to take them away.”

The couple didn’t let the challenge dissuade them from including complicated installations in the new hanging, however. Ai Weiwei’s Tree, 2009–10,one of a pair seen at Berlin’s Neugerriemschneider during Gallery Weekend 2011, proved the most difficult. “To install a six-meter-tall tree in the middle of a bunker where the doors are only one meter wide, it was necessary to cut out ceiling sand walls, which we then had to close back up and paint over so that you wouldn’t know,” says Christian, pointing out that the work’s upper branches come within inches of the walls. Diamond tipped saw blades aside, the placement of that particular piece was further complicated by the couple’s stringent policy that the artist must personally install his or her work in the bunker. The creative problem-solving to get around Ai’s current Chinese government-imposed travel restrictions ultimately involved Skype and a trip to Beijing made by the Boroses last February to meet with the artist.

If the second hanging differs markedly from the first, it’s in the more introverted mood of the works on view. Many large installations are again in the mix, but
 there is less flash, and the works don’t give themselves over to the viewer as easily. Pieces like Klara Lidén’s Teenage Room, 2009, a uniformly black ensemble of charred bedroom furniture, replace shiny paintings and installations by Anselm Reyle, who figured heavily in the first show. Olafur Eliasson, a Boros favorite with more than 30 works in the collection at large and 14 in the first show, has only three pieces in the second hanging. His brass-and-mirror Orientation Star, 2009, is paired with and reflects Colour Experiment No. 10, 2010, while Driftwood Family, also from 2010, spreads out from the doorway of one room, as if a 100-year flood has managed to penetrate the bunker’s nearly 10-foot-thick walls.