Polar ice is melting. That much is for sure. How it’s melting is what scientists want to figure out. They suspect Antarctica’s melting glaciers are doing so from the bottom up, and they looked to robots for the definitive answer.

Specifically, scientists were looking at Antarctica’s ice shelves – not sea ice.

The California Institute of Technology led an international study that used three underwater robots, or gliders. Each of them measure about six and a half feet long and were designed to measure temperature and salinity of the ocean water in the Weddell Sea, just off the coast of Antarctica.

Here’s what the gliders found. Circular movements of water, also known as eddies, pushed heat into the waters around Antarctica, leading to coastal ice melt. These eddies aren’t small either. They range in size from around a kilometer to 10 kilometers. Plus, they are hard to predict.

“Eddies are instabilities that are caused by ocean currents, and we often compare their effect on the ocean to putting a spoon in your coffee,” lead author Andrew Thompson said.

“If you pour milk in your coffee and then you stir it with a spoon, the spoon enhances your ability to mix the milk into the coffee and that is what these eddies do. They are very good at mixing heat and other properties.”

The study’s findings support previous theories that warm water, not air, was the culprit for Antarctica’s melting ice shelves.

“When you have a melting slab of ice, it can either melt from above because the atmosphere is getting warmer or it can melt from below because the ocean is warm,” says Thompson.

“All of our evidence points to ocean warming as the most important factor affecting these ice shelves, so we wanted to understand the physics of how the heat gets there.”

The team’s robot gliders used an ingenious way of propulsion. Instead of a propeller, which would need a lot of energy to run, the team used buoyancy. A water pump altered the internal buoyancy of the glider. Pump water in, and the glider drops. Pump it out, and it rises. A pair of wings translated the vertical movement into horizontal movement. Here’s a video showing how it works.

Oceanographic Underwater Glider Animation from Sunke on Vimeo.

The next step for researchers will be figuring out what happens when this warmer water hits the area right where glaciers go from the land to the sea.

Thompson and his team plan to use the ocean gliders in the Drake Passage (an area between the southern tip of South America and Antarctica) next.