[reaches out to grab hold of it]

It's warm in your hand, like a smooth, living wire. You still have no urge to breathe, no sense of pressure building in your lungs.

[gently tugs at the thread]

It gives a little when you tug, but that's all; it goes a long, long way down. If you're not swimming against it, the current is still pulling you down in the same direction the thread is going.

[....I suppose I'll be trying to swim all the way down.]

You continue down until your feet touch stone, strangely flat and even for the bottom of the sea, and the current continues to push you forward. You begin to perceive a dim glow in front of you, which grows rapidly closer, until you reach the mouth of a tunnel that slopes up.

[...I'm spending a great deal of time just allowing the current to push me around.]

[But I'll be going into that tunnel.]

There's some kind of bioluminescence on the walls of the tunnel, but as you go up you see that the real source of light is coming from up ahead, getting brighter as you approach it.

[quickens his pace and hurries toward the light]

After a short swim upwards, you break through the surface in a small, dimly lit cave, with a sloping white sand beach. The source of the light is a brilliantly glowing, foot-high spool of fine silver thread -- the source of the thread you've been following. The cave is otherwise empty except for a doorway obscured by a thick grass curtain.

[...kicks the spool.]

It tips over and splashes into the water. The walls shake.

[...now would be a great time to leave through the doorway, won't it?]

Probably. The shaking stops quickly enough, though.

If you go through the door, you find yourself here]