*The door is a preserved artifact of the people who made the four-toed statue and the ruins.
*The door was made by the same people who built The Temple.
*The entrance was linked to the secret room after the purge, what we have seen of DHARMA suggests that they had no idea the barracks was built above ancient tunnels.
*The water is drained into The Source, channeling the water and the light as described by the Man in Black in Across the Sea.
*It is highly possible that the room behind the door is a minuature model of the source for example there is a cork in the center of the puddle of water just like the source's giant cork and if you unplug it the smoke is unleashed the same way if you unplug the source's cork all evil is unleashed thus causing the Island sinks and the light goes out.