a·toll (ătôl′, -ŏl′, ātôl′, ātŏl′)
An island or chain of islets connected by a coral reef that nearly or entirely encloses a lagoon.
[Dhivehi atholhu; probably akin to Sinhalese ätul, interior (from the interior lagoon), probably from Sanskrit *antala-, interior, variant of antara-; see en in the Appendix of Indo-European roots.]
(click for a larger image)atoll
An atoll is created as a coral reef builds up around an eroding volcanic island.
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