Panetta said U.S. technical experts are still assessing available data to learn more about the underground explosion.

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"Well No. 4 in this area was being drilled at this site here when suddenly an underground steam explosion managed to get its way to the surface, " he explains.

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Shatter cones are surface features with distinctive wavy patterns that are known to be created only by the tremendous force of a meteorite impact or an underground nuclear explosion.

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In 1993, an early-morning explosion in Managua below the workshop of a Basque mechanic revealed an underground bunker stuffed with weapons, forged passports, and documents, including plans for past kidnappings to raise money, notably that of a Brazilian industrialist, Abilio Diniz, in 1989.

ECONOMIST: Nicaragua