TNT Barrels are a natural environmental object, commonly found in The Cellar and The Caves/Catacombs. When hit by tears, they become increasingly bloated and eventually explode when struck again. This functions as a typical explosion and can damage Isaac or enemies, break rocks, form bridges, etc. They are also known to occasionally drop a Mr Boom item.
- Buttlickers (commonly found in Catacombs) will detonate a TNT Barrel if they run into one.
- If the player has Wrath of The Lamb installed, TNT Barrels may sometimes be found stuck inside walls. This is only a graphical glitch, so the Barrels cannot be destroyed.