Vault 120

Vault 120 is a Vault-Tec vault located underwater in the Commonwealth. It would have been involved in the cut quest 20 Leagues Under the Sea.

Background
The vault was placed underwater, based on untextured assets that can be found in xEdit of both an underwater vault window (VltSquidWindow01) and an underwater vault door (VltLGDoorMarine01). Based on these unused cells the vault would have included an airlock. Vault 120 would have been split into four levels plus an undersea level, the first level is under code-name "Stellwagen Gorge," while the undersea level is under the name "Vault 120 Research Station"

Behind the scenes
Todd Howard discussed Vault 120 and the 20 Leagues Under the Sea quest in a Reddit AMA on November 10, 2021. He stated that it was to be a "Bioshock style vault," with a giant sentient octopus outside.