Until now the shrimp-like crustaceans were thought to only live within several hundred meters (yards) of the ocean surface, the study said.
The discovery radically changes the scientific understanding of the major food source for marine animals including fish, squid, penguins, seals and whales, said the study, published in the journal Current Biology.
"Most krill make their living in the ocean's surface waters," said Andrew Clark of the British Antarctic Survey.
"It was a surprise to observe actively feeding adult krill--including females that were apparently ready to spawn--close to the seabed in deep waters."
Antarctic krill feed on tiny phytoplankton and live in schools, called swarms, sometimes reaching densities of 10,000 individuals.
A key link in the Antarctic food chain, they grow to lengths of six centimeters (2.4 inches) and weigh up to two grams (0.7 ounces). They have a lifespan of up to eight to ten years.Researchers from the British Antarctic Survey and the National Oceanography Center in Southampton used a deep-diving, remotely operated vehicle to film the krill in the nearly pitch-black depths.