An atoll is a midocean coral reef, roughly circular in shape or sometimes open-ended like a horseshoe, that encloses a lagoon. An atoll—the term originates in the Maldive Islands—may be many miles in circumference and may be surmounted by or, more rarely, surround low sand and coral islets. Before he had ever seen an atoll, Charles Darwin, while aboard the Beagle, relied on ship captains’ charts and descriptions of oceanic islands to come up with a beautifully simple theory of atoll formation. Whereas volcanic islands wear down and subside after eruptions cease, Darwin surmised that their fringing coral reefs continue to grow. These reefs will remain, he argued, if their upward growth can keep pace with islands’ subsidence and if sediments accumulate on or within them to form sand and coral islets. (It wasn’t until the U.S. Navy drilled more than a mile deep in the coral formation on Eniwetok Atoll in the Marshall Islands in 1952, reaching the atoll’s submerged basalt foundation in preparing for the world’s ﬁrst hydrogen bomb test, that Darwin was proved right.) Kure Atoll in the Hawaiian archipelago is the northernmost atoll in the world. It lies close to what is called the Darwin Point—the latitude at which coral growth can no longer keep up with an island’s sinking under its own weight.