A cay (pronounced “key”) is a small, low island made mostly of coral or sand, usually found in tropical waters. These tiny islands dot the Caribbean Sea and other warm ocean areas, often appearing as rings of white sand surrounded by clear blue water.

Most cays form on top of coral reefs over thousands of years. As waves break apart coral and shells, the pieces pile up until they rise above the water's surface. Some cays are barely bigger than a baseball field, while others stretch for miles. Many remain empty except for seabirds and crabs, but some support small forests of palm trees and other salt-tolerant plants.

The Florida Keys are a famous chain of islands that include many cays, stretching from the southern tip of Florida toward Cuba. Pirates once used these islands as hiding spots, and today they're popular destinations for snorkeling and fishing.

Because “cay” sounds exactly like “key,” you'll sometimes see both spellings used for the same kinds of islands, especially in Florida, where places like Key West share the same origin.