Dramatic photos this week from a Russian expedition that descended into a mysterious crater in Siberia, where scientists battled temperatures of 12°F (-11°C), provoked some Internet commenters to quip that "hell really has frozen over."

Scientists from the Russian Centre of Arctic Exploration donned climbing gear and entered the 54-foot-deep (16.5 meters) crater, which was discovered in July on Siberia's remote northern Yamal Peninsula.

The Russian Centre could not be reached for comment, but expedition leader and center director Vladimir Pushkarev told the Siberian Times that it was the first time the team could safely visit the huge hole, as it had been stabilized by the cold.

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The scientists took various measurements and plan to provide more information soon on what may have caused the crater, which made headlines around the world when it was discovered. Many have speculated on its causes, often linking it to melting methane hydrate—an ice-like material frozen in the Arctic ground—though Pushkarev says it is too early to say.

—By Brian Clark Howard, photo gallery by Sherry L. Brukbacher

Pictures: Explorers Descend Into Siberia's Mystery Crater

Scientists investigate a mystery crater that opened on Siberia's Yamal Peninsula this past summer.