When you have placed all four TRIDENTS on the statues, the deep pit at the JUNCTION will be flooded about halfway to the top.

BUG NOTE: If you have placed all the TRIDENTS and the pit still isn't flooded, try returning to each of the side rooms and making sure all the skeletons have been dealt with. That should fix the problem. If you are missing any of the tridents, check the note at the beginning of the main walkthrough.

Dive into the flooded pit and swim down through the passage on the west wall...

...then through the narrow opening at the bottom of that passage on the left.

This leads to a room where you can surface...

...and climb out of the water.

Vault up onto the ledge directly ahead. Two sets of stairs lead up to the left and right. Follow either one. (Here we'll take the stairs on the left.)

When you reach the small room at the top, a skeleton will arise from the floor.

If you then go back down the other set of stairs to the room with the pool...

...and then roll to face the stairs you came down...

...another skeleton will appear behind you. Take it out with explosives, etc.

Then continue up the stairs to the next room.

[Temple of Poseidon Walkthrough]