The Thief's Downfall is a waterfall that pours over the tracks to the Gringott's vaults. It washes away all enchantments and magical concealments.

From the Story

Discovered in Book 7, Chapter 26, Gringotts

The Thief's Downfall is a defence set off by Gringotts employees once they become aware of imposters in the bank. It washes away Hermione and Ron's disguise, and lifts the Imperius Curse from Bogrod the goblin.