dark depression

Astronomers managed to examine closely mysterious “dark depression” on Mars’ surface, which turned out to be deep stone wells. A high resolution camera (HiRISE), established on board the Mars Reconnaissance Orbiter spacecraft revolving around Mars, has received new pictures of mysterious ” fathomless depression” on the planet’s surface. In the previous photos it looked like a black spot in the size approximately 100×150 meters. It was not visible any hint on a bottom, walls or an external swell which form a kind of a classical impactive or volcanic crater. New pictures are made in beams of the sunset which highlights one part of the “well”. Now it is obvious, that it is a vertical wall, it is not revealed signs of a bottom at least up to depth of 80 meters. By the present moment it was possible to find only seven such depressions.
Do you have any assumptions, what it could be???