At dawn, the boys set out on their journey equipped with their new weapons. Ralph describes their different patterns of marching, but he reflects that he felt a unique contentedness being on the island which he later associates with religion. After rounding a bend, they investigate the rugged area where the reef nearly meets the island and discover water spouts erupting violently from holes in the rock, which drench all three of them in turn. After drying their clothes by a fire, they deduce that large waves force water through underground channels to create the spouts. Jack then spots a faint pale-green object with a moving tail beneath the water. They attempt to spear it but miss, and unable to identify it, they continue their journey. Ralph plans to revisit the location later.

The boys explore a small valley and discover edible plants: taro, yams, and a sweet potato-like root. In the island’s largest valley, Jack identifies a “banian,” or banyan, tree, which Jack recognizes from a book, and a chestnut tree with natural plank-like buttresses, realizing they can harvest ready-made planks for building.

Heading toward the coast to camp, Ralph kills a wood-pigeon with his sling. Peterkin goes to shore to build a fire. Jack and Ralph follow wild ducks to a beautiful inland lake, where they find a large tree bearing plum-like fruit, beneath which about twenty hogs sleep. They wound a small pig, but it tears free and escapes with the herd.

Returning to camp, they find Peterkin missing, but they hear him shouting “Hurrah” in the distance. He appears carrying the escaped pig on his spear, having killed it himself. He also reports finding a square patch of sugar-cane, suggesting past human inhabitants. Jack thinks “savages” lived on the island at some point. They prepare a feast of roast pork, pigeon, and the new vegetables.

Continuing their journey, the boys hear the mysterious cry again and identify the source as penguins on a nearby island. Ralph reflects that certain trees grow in specific areas, but coconut trees grow everywhere on the island, including on the reef. Climbing the mountain, they find coral and shells at the summit, leading them to theorize the island was once submerged.

On the third day, animal tracks lead them to what appears to be a wild cat. When Jack’s arrow misses, the animal approaches slowly, and they realize it is old, blind, and nearly deaf. It responds to Peterkin’s gentle voice with affection, and they conclude it once belonged to a human.

The track leads to a clearing with old tree stumps showing signs of past human activity. They discover a dilapidated hut containing only a stool, a rusty pot, and a low bedstead on which lie two skeletons belonging to a man and a dog, with the dog’s head resting on the man’s chest. They find an old axe, an iron pot, and a pistol but no identification. They collapse the hut over the skeletons as a burial mound and return to camp, though they worry that they might meet the same fate as the dead man.

After their journey, the boys sleep for nearly a day and a half. Ralph finds his tank’s creatures dead and the water putrid. Jack explains that evaporation has made the water too salty and advises Ralph to add live seaweed and sand while maintaining proper salinity by replacing evaporated water with fresh water. They watch one surviving crab molt, emerging soft but larger from its old shell.

Ralph successfully implements Jack’s advice, and the tank thrives. For many days, while Jack and Peterkin build a boat using the chestnut planks, Ralph studies his tank with the burning-glass, observing anemones capturing prey, coral insects building, barnacles feeding, hermit crabs changing shells, and an animal that can eject and regrow its stomach.

Three weeks later, Peterkin grows restless and Ralph suggests investigating the mysterious green object at the spouting rocks, which they call Spouting Cliff. Peterkin throws his spear through it without effect. Jack dives down to investigate and disappears. After several minutes without resurfacing, Ralph and Peterkin fear he has drowned. Jack reappears triumphant and explains that the green light comes from an underwater passage leading to a large cave where he could surface and breathe.

Jack and Ralph prepare a waterproof torch, dive back into the cave, and light it to reveal a vast cavern with gleaming walls and large mineral pillars. They explore until their torch burns low, and determine that the green light is reflected by a large coral formation near the entrance.

Walking home, Jack and Ralph describe the cave, Diamond Cave, to Peterkin. Ralph notes the island’s peculiar tides and the absence of twilight, with darkness falling immediately after sunset.

They hear pigs nearby. Jack circles behind to drive the herd toward Ralph and Peterkin waiting at a gorge. A small pig collides with Peterkin, and Ralph kills it with his sling. When the main herd charges through, Peterkin ignores the smaller pigs and spears an enormous sow, killing it instantly. He explains he wants her hide to make shoes. They carry both animals back to camp and feast.