They have beaches made of coral sand, and perhaps a few palm trees growing on little bars of coral rubble. The center of the atoll is a shallow lagoon. The island bit of the atoll is a narrow ring of land around the lagoon, with coral reefs all around the outside of the island.
Atolls generally rise up out of very deep water.
An atoll may be a simple ring of island, or it may be a ring of several islands around a lagoon. In the early 1800s, once it was realized that corals were animals that grew their skeletons only in shallow water, atolls became puzzling features of the tropical oceans. Reef building corals have symbiotic algae that grow in their tissues and help them secrete a calcium carbonate skeleton. They can only grow in the upper 100 meters or so of the ocean where light penetrates. Corals are the main building stuff of coral reefs and are bound together and cemented by algae algae, which also require sunlight to grow.