Poseidon Tridents
From WikiRaider
The Poseidon Tridents are needed to fill the deep pit with water in the Temple of Poseidon, in order for Lara to swim down and access The Lost Library. There are four fountains in the temple with a statue of Poseidon at each. The trident at the end of his staff is missing at each statue. Once Lara places the trident back on the Poseidon Statues, the fountains water is released which flows into the central deep pit. All four tridents can be found in the Catacombs.