The world’s oceans are like an alien world. The National Oceanic and Atmospheric Administration estimates that 95 percent of them remain unexplored. But the mysteries do not start a mile below the surface of the sea. They start with the surface itself.

Scientists are now discovering that the top five-hundredth-inch of the ocean is somewhat like a sheet of jelly. And this odd habitat, thinner than a human hair, is home to an unusual menagerie of microbes. “It’s really a distinct ecosystem of its own,” said Oliver Wurl, of Canada’s Institute of Ocean Sciences.

This so-called sea-surface microlayer is important, scientists say, in part because it influences the chemistry of the ocean and the atmosphere. “One of the most significant things that happens on our planet is the transport of gases in and out of the ocean,” said Michael Cunliffe, a marine biologist at the University of Warwick in England. The ocean stores a large fraction of the global-warming gases we produce; at the microlayer, the gases are pulled down.

“It’s the ocean breathing through its skin,” Dr. Cunliffe said.

Sailors have long known that the surface can be covered with oily slicks (hence the phrase “pouring oil on troubled waters”). But when scientists began studying the surface in the mid-20th century they found it vexing. A scientist cannot just dunk a bucket into the ocean without dredging up deeper water as well. “Even defining the surface is hard, since it’s moving up and down,” said Peter Liss, a professor of environmental sciences at the University of East Anglia in England.