Between the volcanic layers are several beds of sedimentary rocks representing deposition in a lake.

The sedimentary and volcanic layers are folded and broken, and form meridional bands.

Probing of the bottom with sound waves, however, showed several "windows" where these volcanic layers were thin, perhaps making it possible to reach the original crust beneath.

The fertile soil consists of deposits of volcanic ash and rich layers of organic matter, thought to sustain the worm during dry seasons.

Herto Bouri is a region of Ethiopia under volcanic layers.

Then the volcanic layer could have been chewed up by subsequent asteroid impacts.

"They conclude the fossils are much older than a 104,000-year-old volcanic layer and very close in age to a 196,000-year-old layer", says Brown.

The first hit was some "Mars jewels" they found in the volcanic layers dotted around Gusev Crater.

A rival date of "2900 BP" or c.1000 BC has since been proposed by scientists based on a re-examination of the volcanic layer.

Reflections due to volcanic layers are possible at any depth.