NASA's Mars Reconnaissance Orbiter (MRO) continues to probe the red planet. Utilizing its High Resolution Imaging Science Experiment (HiRISE) equipment, the spacecraft has focused in on dark holes on some of the martian volcanoes.

Speculation has centered on these features being entrances into caves. One earlier HiRISE shot stared straight down into one of these oddities, spotting only darkness in the pit.

A subsequent flyover of that same locale imaged the feature from a different angle – but this time the Sun was shining from the west. What shows up this go-round is an eastern wall of the pit.

According to Mars experts, the new image confirms that this pit is essentially a vertical shaft cut through the lava flows on the flank of the volcano. Such pits form on similar volcanoes in Hawaii and are called "pit craters."

Back here on Earth, such pit craters don't generally connect to long open caverns – but are the result of deep underground collapse. From the shadow of the rim cast onto the wall of the pit, specialists calculate that the pit is at least 255 feet (78 meters) deep. The pit is 492 x 515 feet (150 x 157 meters) across.

--Leonard David

Credit: NASA/JPL/University of Arizona