New images from Mars, taken by NASA's Mars Odyssey spacecraft and the Mars Global Surveyor have shown what appears to be at least seven entrances to large caves on the slope of Arsia Mons, a Martian volcano. Scientists believe the caves are very large and they believe that they lead to the subsurface of Mars.
All of the cave entrances, which were named "The Seven Sisters", are at least 100 to 250 meters (328 to 820 feet) in diameter.  Infrared images show that the temperature variations from night and day show that they are likely caves.  "They are cooler than the surrounding surface in the day and warmer at night. Their thermal behavior is not as steady as large caves on Earth that often maintain a fairly constant temperature, but it is consistent with these being deep holes in the ground," said Glen Cushing of the United States Geological Survey (USGS) Astrogeology Team and of Northern Arizona University located in Flagstaff, Arizona.
While some scientists say that "somewhere on Mars, caves might provide a protected niche for past or current life, or shelter for humans in the future" (Tim Titus of the U.S. Geological Survey), others disagree as to this possibility, since most of the caves "...are at such extreme altitude, they are poor candidates either for use as human habitation or for having microbial life. Even if life has ever existed on Mars, it may not have migrated to this height," added Cushing.
The caves are believed to have been formed as underground stresses around the volcano caused spreading and faults that opened spaces beneath the surface. Some of the holes are in line with strings of bowl-shaped pits where surface material has apparently collapsed to fill the gap created by a linear fault.
NASA plans to use the Odyssey to find more possible caves at lower altitudes in the future.

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Seven caves found on Mars: NASA