The tunnel continues onward with no end in sight. Your lungs heave with pain. You have held your breath far longer than you ever thought possible. For a moment you wonder if you shouldn't turn back, but by now the entire cavern must be completely submerged. Even if it weren't, you couldn't hope to outlast the creatures that dwell inside for very long. No, there is no going back. Your only hope is to press forward.
Another minute passes. The current seems to grow stronger as the tunnel narrows ahead. Your chest feels as though it is about to burst. Finally, you emerge from ice-cold seawater and tumble out of the cavern, landing hard on a rocky shore.