A teen school project on Mars uncovered a surprise - what appears to be a hole in the roof of a cave on the red planet, according to researchers.

The 16 students in a seventh grade science class at Evergreen Middle School in Cottonwood, California, chose to study lava tubes, a common volcanic feature on earth and Mars as their class project sponsored by Arizona State University's Mars Education Programme.

The students developed a research project focused on finding the most common locations of lava tubes on Mars, according to their teacher Dennis Mitchell.

While studying the thermal images from a Mars-orbiting camera they also found a small, round black spot, which appeared to have vertical sides, on the Pavonis Mons volcano.

Glen Cushing, a US Geological Survey scientist, suggested after a similar discovery in 2007 that the anomalous pit craters are "skylights" - places where a small part of the roof of a cave or a lava tube had collapsed.

"This pit is certainly new to us," Dr Cushing told the students. "And it is only the second one known to be associated with Pavonis Mons."

He estimated it to be approximately 190-by-160 metres wide and 115 metres deep at least.

The students have submitted their site as a candidate for imaging by the High Resolution Imaging Science Experiment camera on Nasa's Mars Reconnaissance Orbiter, according to a statement from the university.

"The Mars Student Imaging Programme is certainly one of the greatest educational programmes ever developed," Mr Mitchell said.

"It gives the students a good understanding of the way research is conducted and how that research can be important for the scientific community. This has been a wonderful experience."