The room is appropiately named after the god of the sea in Roman mythology (also known in Greek mythology as Poseidon), due to the task set out to retrieve the Neptune key.
When you enter the room you find yourself in a small chamber with a small pool in its center. There is also an ante-chamber connected alongside the central room.
The task requires the individual to dive into an extremely deep tunnel of water. Within the deep tunnel of water the individual must find a lever which once activated will open a gateway to an underwater ante-chamber (underneath the Neptune room ante-chamber) that holds the Neptune Key.
The aim is to swim down the underwater tunnel and retrieve the key by holding your breath within the time frame of two minutes, Lara can only hold her breath for two minutes.