A team of scientists led by the University of Leeds has used a robotic 'yellow submarine' to observe detailed flows within an 'undersea river' for the very first time. The researchers estimate that the river - known as a submarine channel - would be the sixth largest river in the world if it were on land based on the amount of water flowing through it. Submarine channels are similar to land rivers, but they are formed by density currents - underwater flow mixtures of sand, mud and water that are denser than sea water and so sink and flow along the bottom. These channels are the main transport pathway for sediments to the deep sea where they form sedimentary deposits. These deposits ultimately hold not only untapped reserves of gas and oil, they also house important secrets - from clues on past climate change to the ways in which mountains were formed. It is thought by some that the channels in the Black Sea were formed around 6,000 years ago when sea levels were approaching their current point. The Mediterranean swelled and breached through into the Black Sea - once an isolated freshwater lake - via the Bosphorus Strait. As the waters surged, they carried a dense, salty fluid which formed a network of sea-floor channels that are almost constantly active, even today.