The Amazon river is known to be the second longest in the world, shorter only than the Nile but, remarkably, scientists have discovered another river flowing thousands of feet beneath it.
Researchers from Brazil's National Observatory believe the subterranean river is 3,700 miles long, about the same length as the Amazon on the surface.
The great Amazon: Now scientists say another river is flowing 13,000ft beneath it
Dr Valiya Hamza, from the BNO, said the discovery of the underground river came from studying temperature variations at 241 inactive oil wells drilled in the 1970s and 1980s by Brazil's state-run oil company, Petrobras.
He said the 'thermal information' provided by Petrobras allowed his team of researchers to identify the movement of water 13,100ft under the Amazon River.
Their findings were presented last week in Rio de Janeiro at a meeting of the Brazilian Geophysical Society.
Computer simulations presented by doctoral student Elizabeth Pimentel, found the groundwater flow is mostly vertical to about 6,500ft deep, but changes direction and becomes almost horizontal at greater depths.
The Amazon River delta (mouth) in Brazil as captured by Nasa's Landsat GeoCover Program. The river is believed to be 11million years old
The apparent underground river has been named after Hamza, honouring the scientist who was the head of the research team that found the signs of the flowing water.
It is believed to start in the region of Acre, flow through the basins of Solimoes, Amazona and Marajo and reach the sea at Foz do Amazonas. This would explain why large pockets of the sea in this area have low salinity.
The average flow of the newly discovered 'Rio Hamza' is just two per cent of the Amazon, but this puts it on par with the San Francisco river in California.
The average flow of the Amazon River is estimated at about 133,000 m3 / s, while the flow of the Rio Hamza is far slower at an estimated at 3090 m3 / s.
Dr Hamza said the existence of an underground river that also flows west to east would mean that the Amazon rain forest has two drainage systems - the Amazon and Hamza rivers.
He stressed that the studies examining the underground river were still in their preliminary stage but added that he expected to confirm the subterranean flow by the end of 2014.
He declined to comment on the economic and environmental impact of an underground river in the Amazon rain forest.
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