The hole measures about 330 feet (100  meters) in diameter and is  located to the northeast of Arsia Mons, one  of the largest volcanoes on  Mars.A few years ago, a football field-sized, dark and very deep hole was discovered by NASA’s Mars Reconnaissance Orbiter.
Is it an opening to a subsurface cave, or other much more deeper realms of Mars?
Later, more similar holes were discovered.
Scientists think the holes could be entrances to underground Martian caves.
Are these giant holes leading to a secret underground world on Mars?
The hole measures about 330 feet (100  meters) in diameter and is located to the northeast of Arsia Mons, one  of the largest volcanoes on Mars.
 Black spots have been discovered on  Mars that are so dark that nothing inside can be seen. Quite possibly,  the spots are entrances to deep underground caves capable of protecting  Martian life, were it to exist. The unusual hole pictured above was  found on the slopes of the giant Martian volcano Arsia Mons. The above  image was captured three weeks ago by the HiRISE instrument onboard the  Mars Reconnaissance Orbiter currently circling Mars.Credits: Credit:  NASA, JPL, U. Arizona 
According to experts studying the MRO’s  images, the feature is not an impact crater; it lacks a circular, raised  or ejecta. The hole’s darkness is impenetrable because of its  sufficient depth, so any details of the terrain inside it, cannot be  seen.

 THEMIS’s image shows holes, which  can be entrances to underground caves on Mars. Photo Credits: GE  Cushing, TN Titus, JJ Wynne, USGS, USGS, Northern Arizona University,  and PR Christensen of Arizona State University 
Similar  holes, dark, impenetrable to sunlight and having the size of a football  field have been discovered on Mars by the NASA Mars Odyssey and its  Thermal Emission Imaging System (THEMIS).
They are located on the flanks of Arsia  Mons, southernmost of the massive Tharsis ridge shield volcanoes, a  region with widespread collapse pits and grabens which may indicate an  abundance of subsurface void spaces.
There are several unanswered questions.
What are these giant sinkholes?
How were they created?
 Are they entrances to a secret underground world where life could exist? 
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