Life exists in the Marianas Trench, the deepest point in the world's oceans and once considered too hostile an environment for life to survive, scientists say.

The 7-mile-deep underwater chasm in the Pacific Ocean with near-freezing temperatures, immense pressures and complete darkness is teeming with microbes, an international team of researchers reported.

"The deepest parts of the deep sea are certainly not dead zones," study researcher Robert Turnewitsch of the paper from the Scottish Association for Marine Science, told the BBC last week.

An analysis of the levels of oxygen in a sample of sediment from the floor of the trench by a deep sea robotic lander revealed the presence of a large number of microbes, researchers said.

"These microbes, they respire as we do," Turnewitsch said. "And this oxygen consumption is an indirect measurement of the activity of the community."

The microbes exist on dead plants and creatures drift down from the sea surface to settle within the steep walls of the trench, he said.

"The amount of food down there and also the relative freshness of the material is surprisingly high -- it seems to be surprisingly nutritious," for the microbes, he said.

The research has been published in the journal Nature Geoscience.