The Underground must sneak through the sewers and crack 10 locks to open a vault. When the vault opens, you may choose to take the loot now, or attempt another heist for even better rewards.
(3) Crack 2 vault lock after a player combat win, and 3 after a player combat loss
(4) Crack 2 vault lock after a player combat win, and 4 after a player combat loss
(5) Crack 3 vault lock after a player combat win, and 5 after a player combat loss
(6) Crack 4 vault lock after a player combat win, and 7 after a player combat loss