The river cascades through steep-sided gorges and churns around isolated towers of rock, before winding across a vast plain beyond. It is a torrent to rival the mighty Colorado River that carved out the Grand Canyon.

Yet this dramatic natural wonder has never appeared in any tourist photographs, nor does it feature on any maps. The reason? It lies two miles (3.2km) beneath the surface of the Pacific Ocean.

Stretching away from the coast of California, the Monterey Canyon has been shaped over millions of years by this bizarre “undersea river”. Beyond the mouth of the canyon, the flow has cut a valley into the sea floor that extends for nearly 200 miles before spilling onto the abyssal plain of the deep ocean.

Similar channels can be found etched into seabeds all over the world. They have been found off the coasts of Greenland, the Amazon, the Congo and Bengal, to name a few. The largest are several miles wide and run for thousands of miles out into the ocean depths, where they provide vital sustenance to the creatures living there. But these undersea rivers are among the least understood phenomena on our planet.