Diana Northup and fellow cave researcher Jenny Hathaway stand under a skylight in the Gruta dos Balcões, a cave in the Portuguese Azores islands. Dark holes that could be similar cave skylights have been found on the moon and Mars, offering the tantalizing hint that such caves might harbor traces of past extraterrestrial life.

In her studies of microbe waste in lava tubes, "Diana is essentially providing a field guide as to what you might find in these things," said the New Mexico Institute of Mining and Technology's Boston.

"It's very clear from our work in all different kinds of caves on this planet that the interior of a cave can be radically different from the external environment," she added. "That might be the case on Mars, as well."

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