atoll (n.)

"island consisting of a strip or ring of coral around a central lagoon," 1620s, atollon, from Malayalam (Dravidian) atolu "reef," which is said to be from adal "closing, uniting." Watkins writes, "Perhaps ultimately from Sanskrit antara-, interior" (from PIE root *en "in"). The original use was in reference to the Maldives. Popularized in present form by Darwin's writings.