"It's a nightmare," said Emily Ansenk, director of the Kunsthal museum in Rotterdam, Netherlands, after an art theft during the night of October 16. She still cannot believe it: Seven works by Pablo Picasso, Claude Monet, Henri Matisse, Paul Gauguin, Meyer de Haan and Lucian Freud were stolen - all from the Triton Collection of Dutch collectors Willem and Marijke Cordia, collected over decades. The perpetrators: unknown.

Moving pictures

It's a nightmare that Ulrich Krempel, director of the Sprengel Museum in Hanover has also faced. In 2008, thieves took two of the museum's Picasso paintings. The works had been on loan to an exhibition in Switzerland, where they were stolen. And yet Krempel had hope: "All the involved parties and investigators said we should expect the paintings would come back, but do not imagine this will happen soon." Finally, after three years, there was a lead. An art detective traced the works to Serbia. Krempel was reunited with the paintings one night in a Belgrade hotel room and could ascertain their authenticity. But the perpetrators remain unknown to this day.

As with the Rotterdam case, Krempel said, there was plenty of speculation - for example, about a billionaire ordering the theft: "I think this is a fiction that is more often found in Hollywood or in crime stories than in real life."

That's because it would not be possible to sell the stolen masterpieces: Both the Picassos from Hanover and the seven paintings from Rotterdam are listed in the Art Loss Register, the largest database of stolen artworks. Anything recorded there cannot be traded by any official dealer, shown at an exhibition or sold at an auction house.

And yet, again and again criminals steal world-class art: The largest art theft in history was the heist at Boston's Gardner Museum in 1990. Two thieves appeared at night at the museum, claiming to be responding to an emergency call. When they were admitted, they tied up the guard. They made off with art worth around 230 million euros. The paintings by Vermeer, Rembrandt, Manet and other masters have never been recovered.

The security concept in Rotterdam is facing hefty criticism. At night, the Kunsthal was only protected by a single alarm system when the burglary happened. It was triggered, but when the police arrived a few minutes later, the thieves were gone along with their booty.

The art world is wracked by uncertainty. Exhibitors such as Roger Diederen, curator of the Kunsthalle der Hypo-Kulturstiftung in Munich, are conflicted: "On the one hand, we want to make art accessible, but on the other hand such incidents are forcing us to turn museums and art galleries into bunkers."

Security costs money, and pushes both small and large institutions to their financial limits. The result: Visitors need to spend more to see art, "Then the audience rightfully asks: Why do I have to pay ten or twelve euros for an exhibition? But very few people understand how very high the overall costs are to transport, insure, and display art. This is an enormous problem."

Another problem: art galleries such as the Kunsthalle Munich have to rely on loans from other museums because they have no collection of their own. If thefts such as the one in Rotterdam force museums to be more cautious, art galleries can no longer organize exhibits because they lack the art.

Art as a visual memory

But Krempel intends to continue to loan art from the collections of the Sprengel Museum, because for him art belongs on public display and not in the museum archives. Art is a kind of visual memory, which only functions if visitors look at and experience artworks "in the best way there is, namely with the head, with vision, with the understanding - and not through theft."

Since the theft of two Picassos he has been looking more closely at the security details in the facility reports of curators requesting loans. But he believes in human goodness: "In general, art is also protected because many people understand art in a wonderful way as our common good."

The two stolen Picassos can now be seen again in the Sprengel Museum in Hannover. Only one suffered slight damage to its frame.