People walked into the fireboxes on ships boilers,too.And they climbed into the engine cylinders of VERY large Deisel engines on the ships-At work out here there are some guys who used to be in the Navy or Coast Guard.And one of them-who was short and thin-he's retired now,but so handy to get into those tight transmitter places----he used to clean the barrels of 16" guns on a battleship.they would tie a line to him---after the gun made SAFE of course---he would climb down the barrel and do his cleaning,inspection.If a gun like that could launch a 1 ton shell at 2,700FPS-imagine what would happen to a person!