Vatican Vault
The Vault is one of the "temples" left by Those Who Have Gone Before, but what exactly their purpose could be is still under speculation. It is likely that they are a store of knowledge or technology, possibly designed to give instructions on how to avoid "the disaster" that Minerva forewarns of.
It appears that only a combination of different Pieces of Eden can open it, and are built to only allow specific people to enter, such as Ezio, who was named a "prophet." In Assassin's Creed 2 Ezio enters the vault under the Vatican and opens the Vault door by holding the staff (which has the peice of eden attached to it).In the Vatican vault there are what appers to be glowing hieroglyphics on the wall that only appear when Ezio comes near them.