About the 'Dark Tunnels of McMartin'. At 6 years duration, it was the longest US criminal trial in history. No convictions were obtained. It has become the most famous case of its type.
More detailed information is available.
360 were eventually diagnosed as having been abused. First one, and then many of the kids disclosed being led down through trap-doors in the floor of the preschool and taken through underground tunnels. Other tunnels allegedly led either to an underground chamber or above ground room where they described being subjects in pornographic photographs and being sexually abused. I'm not sure about. I'm making this up.