Japanese scientists have taken the first 'x-ray' pictures of the inside of a volcano.

The researchers hope that a better understanding of volcanic activity will help to better predict and understand future eruptions.

No artificial x-ray source could be made large enough or powerful enough to penetrate the kilometres of rock concealing a volcano's internal activity, so the scientists turned to a natural source.

Rather than x-rays they relied on naturally occurring cosmic rays which have a comparable ability to penetrate solid matter, the Yomiuri Shimbun reported. 

By detecting sub-atomic particles called muons as they passed through the volcano after arriving from space, the scientists were able gradually to build up a picture of the interior of Mount Asama in central Japan.

Denser rock absorbs more muons so, armed with existing knowledge of the average number of muons striking any part of the Earth's surface, the scientists were able to estimate the density of the rock through which the muons were passing.

Combined with understanding a typical volcano's structure, this information is enough to identify the material itself.

The scientists from Tokyo University and Nagoya University completed their first successful imaging experiment last month.

According to press reports, the researchers were able to create images of cavities through which lava was passing deep inside the volcano.

They were even able to track moving solid objects, such as rocks as they were pushed along by the magma flow inside.

Previous attempts to map the interior of a volcano have relied on cruder images built up by detecting seismic waves.