Craziest hot tub I've ever been in was at a private festival in Girdwood Alaska (the band I was road mgr for was flown out to play there back in the mid 90's. It was a crazy hippy fest for a couple hundred people. They had mushroom smoothie machines....if that tells you anything.
This "hot tub" could sit upwards of 60 people. It was an aluminum trailer of an 18 wheel tanker truck someone had torch cut in 1/2 longways. They lined the bottom w/ pallets and built a row of wooden benches down the length of both sides. (Plus a walk deck on the top of one side w/ steps to the ground) They tapped in a big water pump on one end that went through a filtration system and fed back through the opposite end. The kicker though.....the heater.....two bonfire sized fire pits underneath the damn thing!! Rogue as can be, and it was....amazing!!
Only wish I had taken pictures.