Carthago

In this chapter, we learn that the ship was equipped with "hypobaric chambers" , which are chambers in which oxygen is sucked into a person's body in order to make it more oxygen-rich. We're told that the "chances of survival" are zero, but that's because the "ancient tritons" were responsible for the explosions that put the ship in the middle of the ocean. The ancient tritons are the ones who caused the first two explosions, and they're also the ones that caused the second one. They're going back up, and the battery life on the ship isn't good enough to get them back up to the surface. The only way to get everyone back up is to go back down again, and that'll take two more dives. Tommy will be the fifth person to go down, and he's the only one who's not from the crew. He'll be the last one to go, because the honor belongs to him.