The water's starting to rise up the stairway they just came up now, and a quick flash of a fin and teeth below would seem to indicate that it's not the empty, pleasant type of water.

Atton scrambles up the ladder, through the trap door, and slams it shut, hands glowing briefly as he welds the metal to that around it.

There's a grate nearby. He considers it for a moment, then positions it over the trap door and welds it down, too.

"C'mon," they're left in a tiny, circular room, with one door out. Atton wanders over and opens it. "Let's go."