Lestrange Vault

"...a cavelike opening crammed from floor to ceiling with golden coins and goblets, silver armor, the skins of strange creatures – some with long spines, other with drooping wings – potions in jeweled flasks, and a skull still wearing a crown."

- Description

This vault at Gringotts Wizarding Bank belongs to the Lestranges, a very old pure-blood family. In addition to containing large amounts of treasure, it was the hiding place of Helga Hufflepuff's Cup, one of Lord Voldemort's Horcruxes.

Security Measures
In order to reach this vault, one must be escorted by an employee of Gringotts on a cart, which passes through the Thief's Downfall. A dragon guards the door and will only shrink back at the sound of a goblin's clankers, in anticipation of being painfully punished. With the touch of the employee, the door will open.

Inside, all the treasure has been cursed with Geminio and Flagrante. Thus, everything is hot to the touch and will replicate at a rapid rate if touched, until the potential thief is buried and burned.

Robbery
Around Easter of 1998, Harry Potter, Ron Weasley and Hermione Granger were captured by Snatchers and brought to Malfoy Manor. There, Bellatrix Lestrange panicked at the sight of Godric Gryffindor's Sword among their possessions, as she had believed the sword to be in her Gringotts vault, along with another item Voldemort had given to her. Thus, after the trio escaped the Manor, they planned to break into Gringotts in order to steal the item from the Lestrange vault.

With the assistance of goblin and former Gringotts employee Griphook, Harry, Hermione and Ron broke into the vault and obtained Hufflepuff's Cup.