Below is an excerpt from an article that originally appeared on Science, written by Erik Stokstad.
At first it seemed like the usual, deviously clever attack. Several killer whales were trying to catch a Weddell seal that had taken refuge atop a drifting patch of Antarctic ice. The orcas swam alongside each other, creating a wave that knocked the hapless pinniped into the water. Death seemed certain.
Then something amazing happened: A pair of humpback whales turned up. As the panicked seal swam toward them, a lucky wave tossed it onto the chest of the closer, upturned whale. The whale arched its chest out of the water, which kept the seal away from the charging killer whales.
Video courtesy of BBC Planet Earth, 2012