The distinctive markings are thought to have formed when underground lava flows carved out tunnels Underground lava flows on Mars have carved out channels beneath the surface - and the long-dead volcanoes have left a network of tunnels in which life could thrive. Distinctive \'pit chains\' on the surface of Mars surround the Tharsus Montes volcanoes on the Red Planet - showing that tunnels and cavities have collapsed after the lava flows ceased.
Scientists now believe that remaining tunnels - which may have had running water in their million-year-history - could be tempting targets in the search for microbial life on the Planet.
The thin atmosphere of Mars means that the planet is blasted with 250 times more radiation than Earth - so life on the surface is unlikely.
Subterranean tubes, protected from the deadly rays, could provide a \'haven\' for life - particularly if water was present.
Mars landers have measured surface radiation around 250 times higher than that found on the Earth, and more than double that experienced by astronauts on board the International Space Station.
Any caves associated with the pit-chains may in future provide a possible refuge for astronauts from the harsh surface radiation.
The tubes were detected in  images released from European Space Agency’s Mars Express orbiter, revealing a series of ‘pit-chains’ on the flanks of one of the largest volcanoes in the Solar System.
The images, taken on 22 June 2011, cover Tractus Catena in the Arcadia quadrangle, part of the vast Tharsis region on Mars.
This region boasts a number of huge volcanoes, including the three collectively known as Tharsis Montes. To their north sits Alba Mons, also known as Alba Patera, one of the largest volcanoes in the Solar System by area and volume.
Pit-chains can have a volcanic origin. Lava streaming from a volcano solidifies on the surface, leaving a molten tube of lava running below.  
Once volcanic activity ceases, the tube empties, leaving behind a subterranean cavity. Over time, parts of the roof over the cavity may collapse, leaving circular depressions on the surface.
On Earth, recent examples can be seen on the flanks of Kilauea volcano in Hawaii, while on the Moon, Hadley Rille, visited by Apollo 15 in 1971, is believed to have formed in the same way billions of years ago.
Another scenario for the creation of the unusual features involves groundwater.
On Earth, there are clear examples of similar structures in ‘Karst’ regions – after the German name for a region extending from Slovenia to Italy, where this phenomenon was first studied.
Some of Earth’s most famous examples are the network of ‘cenotes’ on the Yucatan peninsula of Mexico. These deep natural pits form when the surface limestone rocks collapse, exposing the groundwater underneath.
This origin is the most interesting in the context of the search for microbial life on Mars. If there are any cave-like structures associated with the pits, microorganisms could have survived, protected from the harsh surface environment.