Brazilian scientists have discovered an underground river around 4 km below the earth's crust which flows below the Amazon on its west to east path. It is as long (6,000 km) as the second-longest surface river in the world, but a hundred times wider.

The Rio Hamza, named after the head of the team of researchers who found the groundwater flow, represents one of two different draining systems for the deteriorating but still large Amazon rainforest region, a statement this week from Brazil's National Observatory said.

Even though the two rivers cover a similar path they have differences. The underground river flows at a far slower pace and empties into the ocean deep underground.

A team of scientists led by Elizabeth Tavares Pimentel came to these conclusions from studying 241 wells drilled by the state oil giant Petrobras in the Amazon region in the 1970s and 1980s.

"It is likely that this river is responsible for the low level of salinity in the waters around the mouth of the Amazon," the statement said.

Pimentel and co-researcher Valiya Hamza presented their results last week at the International Congress of the Society Brasiliera Geophysical in Rio de Janeiro.

The underground river starts in the Acre region under the Andes and flows through the Solimões, Amazonas and Marajó basins before opening out directly into the depths of the Atlantic Ocean.

The Amazon flows much faster than the Hamza, which inches along at less than a millimetre per hour. However, Hamza conveys a much larger volume of water to the sea than the Amazon and the Nile, the world's longest river, combined.

The researchers used a mathematical model to predict the presence of the underground river, based on the measured changes in temperature down the wells. In the presentation, Piementel said that the flow of groundwater was almost vertical through the rocks to depths of around 2,000m. After this, the water flow changes direction and becomes almost horizontal.

Professor Hamza said Piementel's measurements represented preliminary work on the discovery of the new river, but he expected to confirm the existence of the flow with additional measurements within the next few years.