The world's largest underground ‘ocean' — a water-body about the size of the Arctic Ocean and located 700 to 1,400 km below the ground and extending from Indonesia to the northern tip of Russia — has found its match. Scientists have discovered in Brazil the longest underground river — running for a length of 6,000 km at a depth of nearly 4 km. It flows all the way from the Andean foothills to the Atlantic coast in a nearly west-to-east direction like the mighty Amazon River. The discovery was made public at a recent meeting of the Brazilian Geophysical Society in Rio de Janeiro. The river ‘Hamza,' named after the discoverer, an Indian-born scientist Valiya Mannathal Hamza who is working with the National Observatory at Rio, makes it the first and geologically unusual instance of a twin-river system flowing at different levels of the earth's crust in Brazil. If the slowing down of certain seismic waves caused by the damp spot helped uncover the underground ocean, the unusual temperature variation with depth measured in 241 inactive oil wells helped locate the subterranean river. Except for the flow direction, the Amazon and the Hamza have very different characteristics. The most obvious ones are their width and flow speed. While the former is 1 km to 100 km wide, the latter is 200 km to 400 km in width. But the flow speed is five metres per second in the Amazon and less than a millimetre per second speed in the Hamza.

Several geological factors have played a vital role in the formation and existence of these subterranean water bodies. The underground ocean, discovered in 2007, has been formed when the plate carrying the Pacific Ocean bottom gets dragged and ends up under the continental plate. Water at such depths would normally escape upwards but the unusual conditions that exist along the eastern Pacific Rim allow the moisture to remain intact. In the case of the Hamza, the porous and permeable sedimentary rocks behave as conduits for the water to sink to greater depths. East-west trending faults and the karst topography present along the northern border of the Amazon basin may have some role in supplying water to the river. If the impermeable rocks stop the vertical flow, the west to east gradient of the topography directs it to flow towards the Atlantic Ocean. Unlike the Hamza, the 153 km-long underground river in Mexico's Yucatán Peninsula and the 8.2 km-long Cabayugan River in the Puerto Princesa Subterranean River National Park in the Philippines have come into being thanks to the karst topography. Water in these places drills its way downward by dissolving the carbonate rock to form an extensive underground river system.

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