I grew up near a pretty massive storm drainage system that ran through the city. We would ride our bikes (flash lights taped to handle bars) all through these systems. Occasionally we would explore upstream (areas of the tunnel that didn't seem to go anywhere or make sense). It was easy to get lost for awhile, but as long as you rode 'down stream', you could usually find your way out pretty easily.. and worse case scenario you could pop open a manhole and hope nobody ran you over. Although the manholes were often miles apart, so a quick exit was never assured.
We found graffiti that read 'Turn back or die!' We kept following and found a room where many tunnels ran together with all kinds of 'Satanic' worship, bones and altars, candles and such. It mostly seemed like a sham, as I'd seen a lot of 'pretend' stuff and over the top wanna be goth kids so that didn't scare me much.
We heard something coming from a connecting tunnel, looked over and it was 3 or 4 guys in robes running towards us waving machetes.
I have never ridden a bike faster than that day. It was like playing F-Zero.