Most chitons are "nocturnal", so they feed at night, and hide under rocks during the day. Their main habitat is the intertidal zone. They live on hard surfaces, such as on or under rocks, or in rock crevices. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Others live subtidally. A few species live in deep water, as deep as 6,000 m (about 20,000 ft).