On three occasions in summer 1941, two Dutch officers
managed to escape. Two were caught, but four reached the Swiss border safely.
The Germans were stumped, until they noticed two Dutch officers slip into a
manhole in the park. The Germans had bolted the manhole cover shut, but the
Dutch had managed to get it off and replace it with a glass bolt, which those
hidden within could break by pushing from inside. When the moment was right,
those hidden climbed out and slipped over the wall.