As the Dawn Treader continues to sail east, everyone feels different and strange in a good way. The sun is enormous, the light is very bright, and they don't need much sleep. Every morning the singing flock of white birds flies overhead to eat the excess food from Aslan's table, then flies back.
Lucy notices that the seawater is unusually clear. As she looks into the sea she sees something small and black moving at the same speed as the ship. When a piece of bread that fell overboard floats across it without colliding, she realizes the black thing is underwater. The black thing becomes enormous for a moment, then goes back to being small.
Lucy figures out that the ship has passed over an underwater hill or mountain. The black spot is the ship's shadow, and when the ocean was shallower on top of the hill, the shadow was closer and looked bigger. This means that the water is so clear, she can see to the bottom of the sea.
As Lucy studies the sea bed, she notices paths of silver-grey sand running along the bottom through an underwater forest. As the forest ends, the path changes color and is lined with stones. Lucy realizes that it's an underwater road.
The road begins to zigzag and get larger, and Lucy understands that it's now going uphill in switchbacks.
At the top of the hill, Lucy sees specks moving on the road. They are going to and from a castle or small city made of pearly, ivory-colored material.
The narrator tells us that, after they go back to England, Edmund and Lucy decide that the sea-people must build their cities and castles on top of mountains because their topography is the opposite of that on land. On land, the valleys seem nurturing and safe, and the high mountaintops seem distant and dangerous. Underwater, the valleys are dark and cold and have threatening creatures lurking in them, while the mountains in the shallows are warmer, brighter, and more peaceful.
The ship passes over the underwater city. Beyond it, Lucy sees fifteen to twenty Sea People riding sea horses.
Suddenly a shoal of fat fish swims between Lucy and the Sea People. A different kind of fish that seems fierce swims up from below, grabs one of the fat fish, and dives down again. Lucy realizes that the Sea People are a hunting party. They use the fierce fish like hawks to catch their prey.
The Sea People notice the Dawn Treader and swim up to the surface of the water to check it out. They come very close to where Lucy is leaning over the side of the ship. Their bodies are ivory, their hair is dark purple, and they all wear jewelry and coronets. The King of the Sea People shakes his spear at Lucy.
Drinian and Edmund come up behind Lucy and ask what she's looking at. She shows them.
Drinian tells Lucy and Edmund to turn around with their backs to the sea and look nonchalant. They do, and Lucy asks why. Drinian says that if the sailors saw the Sea People, they might fall in love with the women or the undersea country and jump overboard.
Lucy says that when she was a Queen at Cair Paravel, they knew and were friends with the Sea People. (Remember, in The Lion, the Witch, and the Wardrobe, the Sea People sing at her coronation.) Edmund says that was a different kind – those people could breathe in the air or underwater, while the people they are seeing now can only live underwater.
They are interrupted by the sound of a splash and a cry of "Man overboard!" But it's not a man – it's Reepicheep. Drinian curses Reepicheep, not because he's angry but because he's frightened about what might happen to him.
Rhince turns the ship around and they go back to the place where Reepicheep went overboard. As they approach to pick him up, they see that he's trying to tell them something.
Drinian is worried that Reepicheep is going to tell everyone about the Sea People, so he runs to the side of the ship and uses a rope to haul Reepicheep up himself, waving everyone else away and saying he can handle one mouse on his own.
As Reepicheep climbs up the rope, Drinian leans over and whispers to him not to say anything about the Sea People. But Reepicheep doesn't care about them – he's excited because he has discovered that the water is sweet (i.e. fresh instead of salty).
Nobody knows why this matters until Reepicheep repeats the prophecy that was spoken over him in his cradle, which says that the place "where the waves grow sweet" is "the utter East" (15.36).
Drinian hauls up a bucket of seawater and gives it to Caspian. He confirms that the water is sweet and says it's more like drinking light than water.
Lucy also drinks from the bucket. She says it's wonderful and strong and that she won't need to eat now.
Everyone on the ship drinks the strange, sweet water. For a long time they are quiet, feeling very strong and healthy. They notice that, although everything is getting brighter (because this world seems to be flat and they are getting closer to the sun), the water has made them able to withstand the bright light.
In the evening Drinian wonders aloud how the ship is moving. There is no wind and the sail hangs limp, but the Dawn Treader is still sailing quickly toward the east. Caspian says they must be caught in a strong current.
Edmund points out that if this world really is flat, then they might just be poured over the edge of it. This distresses Caspian, but Reepicheep says that's how he's always imagined the end of the world.
Drinian asks Reepicheep what he thinks would be waiting for them at the bottom after they plunged over the edge of the world. Reepicheep says that maybe it would be Aslan's country, but maybe it would be an endless fall.
Eustace is confused and says the world is round like a ball. Edmund says that just because their world is round doesn't mean that Narnia's is.
Caspian is intrigued by the idea of a round world and asks Edmund whether he has been to the places where people are upside down. Edmund says it's not really like that.