Lassen National Park, Bumpass Hell

Named after R. V. Bumpass who while giving a tour and telling people to be careful, promptly fell through the thin crust and was severly burned by the hot water and steam.  This is one of the main geothermal features at Lassen NP.
Just make sure that you pay attention to the signs and stay on the trails and boardwalks.  It's a 2 mile hike back to your car if you were to fall in and there are no ambulances nearby.