Mt. Bikini

Sometimes the mills of science grind exceeding slow. Recently the U.S. Navy announced that it had corroborated a Darwinian theory 110 years old.

In 1831, when the great, grave, bearded Charles Darwin was a bubbling young naturalist, he began his famous voyage on the Beagle. While crossing the South Pacific, he was fascinated by the ring-shaped coral islands, which he decided to call "by their Indian name of atolls." He wondered about those saucers of coral standing on steep-sided platforms above the deep ocean floor. Why their ring shape? How had they been formed? It was known that reef-building corals did not thrive more than a few fathoms below the surface. Certainly the islands had not grown upward from the depths. The atolls, he concluded, must have been formed when islands sank, and the coral reefs fringing their shorelines continued to grow. "For as mountain after mountain, and island after island slowly sank beneath the water, fresh bases would be successively afforded for the growth of the corals."

Trial by Depth Charge.

Darwin's atoll theory won fairly wide acceptance, but it was not checked conclusively until the Navy decided to explode two atomic bombs at Bikini Atoll last year. During the preliminary survey, scientists mapped the underground structure of the atoll by seismic methods: 126 depth charges exploded at various points on the bottom of the lagoon sent waves through the coral and underlying material. The denser the medium, the faster such waves travel. By measuring how long the waves took to reach listening instruments, the Navy's scientists could estimate the density of the rock at various depths (see chart).

For some 2,000 feet down they found that the atoll was made of coral and similar stuff, rather like what was found on the surface. Then began a zone of heavier rock, which might be ash thrown out by a volcano, or limestone formed by corals and other sea creatures and compacted by pressure. At about 5,000 feet, they found what they had hoped to find: a "buried mountain" of heavy rock.

It might be the hard core of a volcano or peak formed by above-sea erosion. Only deep drilling could give the details. But the mountain was there, far below the growing zone of coral. Darwin, from the deck of his windjammer, had guessed right.

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