The trapdoor is made of wood with a metal o-ring handle on one side. You grip the ring on the trapdoor with both hands and with a bit of strain manage to swing it open. It reveals a stone spiral staircase heading downward. You catch a salty, musty scent rising from the hole and the steps appear damp. The stairwell seems to be filled with the same green light as the room you're currently in.

You can go down, or you can go back and...

Check the first hatch
Check the second hatch
Check the hole in the wall
Check the rope ladder by the wall