Photographs of enormous pits on the moon, some hundreds of feet deep, from unmanned probes have given scientists a tantalizing glimpse into the lunar interior.
Some of the moon holes are wide enough to fit the White House and scientists think they are openings to underground tunnels that had been formed by rivers of lava.
"They could be entrances to a geologic wonderland," said lead researcher Mark Robinson at Arizona State University. "We believe the giant holes are skylights that formed when the ceilings of underground lava tubes collapsed."
First seen in close detail by Japan's Kaguya spacecraft last year, the lunar pits were also seen by NASA's Lunar Reconnaissance Orbiter using same high resolution camera that photographed the lander portion of the Apollo spacecraft and astronaut footprints in moon dust. [10 Coolest Moon Discoveries]
Lunar lava tubes and trails
The existence of tunnels in the moon was first proposed by scientists in the 1960s, when early photographs showed that hundreds of long, narrow channels trailed across the lunar plains.
Taken as evidence of past volcanic activity, the grooves ? known as rilles ? had pointed to the possibility of underground channels similar to lava tubes found on Earth.
Lava tubes form when the upper portion of a river of molten lava cools and solidifies while the rest of the lava continues to stream beneath it. The insulated molten rock can retain enough liquid warmth to flow for miles, carving out tubular channels and complex labyrinths.
Images from Japan's Kaguya spacecraft depict gaping holes on the same plain, or lunar maria, as the winding rilles. One particular pit appears in the middle of the channel, leading scientists to believe it represents the collapsed roof of an underground tube.
Researchers speculate that the tunnels, if unclogged, could serve as passages and livable lunar lairs for humans.
"The tunnels offer a perfect radiation shield and a very benign thermal environment," Robinson said in a statement. "Once you get down to 2 meters under the surface of the moon, the temperature remains fairly constant, probably around -30 to -40 degrees C."
Explorers would be sheltered from daily temperatures that swing from 212 degrees Fahrenheit (100 degrees Celsius) during midday to minus 238 degrees Fahrenheit (minus 150 degrees Celsius) at night, as well as from possible asteroids. But further exploration would be needed before the tubes could be used.
"Hold off on booking your next vacation at the Lunar Carlsbad Hilton," said Paul Spudis of the NASA-funded Lunar and Planetary Institute in Texas. "Many tunnels may have filled up with their own solidified lava."
Viewed though entrances, the blackness of the enormous pits for now remains a tantalizing wall.
"We just can't tell, with our remote instruments, what the skylights lead to," said Spudis. "To find out for sure, we'd need to go to the moon and do some spelunking."
Relaying how a lava flow mapping expedition in Hawaii revealed a surprising system of vents similar to the skylights photographed, Spudis left open the possibility of a lunar labyrinth.
"It turned out that there was a whole new cave system that was not evident from aerial photos ? Who knows? The moon continually surprises me," he said.
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