After we finished looking around the city of Maastricht, we headed about 2 km out of the city to a place where we could see the tunnels of Maastricht. These were dug out because the limestone was used in many ways for building in the city. 20,000 tunnels are dug out. These were also planned to be a hiding place or shelter for people during the war. At the end of the war 2,000 people did seek shelter there, but our guide said it was a failure. No facilities, dark, damp. They were there ten days. Deep in the caves our guide showed us where the Nazis had built a vault with cement walls and air conditioning to hide the famous art work they had taken from around the Netherlands. We went in there. It was eerie. All the artwork was returned to the owners after the war. The map you see on the wall shows major portions of the tunnel system. Our guide said its very easy to get lost and people have been lost for days. He went ahead and let those of us who wanted to, walk in total darkness with our right hand touching the wall for about three minutes. That was an experience.