Explosive Barrels

 are obstacles in Build A Boat For Treasure in the Toxic Waste Stage. Upon anything touching it (Blocks or players), it will explode, dealing damage to boats in a similar fashion to mines. If a player is standing within a radius of a few blocks, the explosion will kill the player, even through solid objects.

Trivia

 * Based on their appearance, they might refer to barrels filled with nuclear waste. This seems fitting to the theme, as there are also green, radioactive leaks scattered around the stage.