Body Toughness training areas are places where players can bathe in dangerous substances to increase their BT. If the player does not have enough BT they will take a severe amount of damage, as a consequence for training there.
Note: While the other powers need the minimum requirement to train in their training areas, you don't have to meet the requirement for Body toughness but you'll need 4x the requirement to take no damage/die in it. Furthermore, you can bathe in these areas without meeting the "requirement", if your Body Toughness is 1/20th of the requirement you will still recieve BT. (Although you'll take much more damage per tick.) Areas can be found at City Port, Tornado, Volcano, and Devil’s Secret Training Area.