There’s nothing cooler than a large storm drain system. They’re fairly common in California, where it’s dry except during the winter rains (and sometimes floods). One of my friends in Oakland once had a map of the entire Oakland storm drain system, which we used to chart a long journey through a covered creek, through backyards and tunnels, up into the Oakland hills. Finally the passage narrowed so much we could hardly turn around, so we emerged from the next manhole we reached, which turned out to be in a cemetery.