Climate change is bad. Melting glaciers are also bad. However, both these things lead to something called "dead water." From a nautical perspective, this is also bad because it stops a ship in the middle of the ocean. But it is so, so cool to look at.
As glaciers melt, they release massive amounts of fresh water into the ocean. Some of this water gets mixed into the seawater immediately. Some of it collects in pools above the existing salty water. The difference in salinity, and the resulting difference in density, keeps the two pools of water separate, with the less-dense fresh water covering the salt water.
A person in a boat going over the ocean shouldn't notice a difference between stratified fresh and saltwater and the regular sea — at first. Then, after a traveling normally for a little while through the layered water, their boat will slow, or even stop, seemingly without cause. As the sailor tries to maneuver the craft around, it will either fail to respond to their steering at all, or exaggerate their turns wildly. It will seem as though the boat is being held back, or pushed around, by powerful forces, but the surface of the ocean will remain calm and unchanging. Sailors who strayed into stretches of water like this call it "dead water."
Recently, scientists simulated dead water in the lab. It was as simple as stacking up water with different levels of salinity, marking the less-dense level with dye, and dragging a toy boat across the surface of the "ocean."
The process is easier to see than to describe. The boat creates a wave in the lower, saltier level of water. This wave isn't visible on the surface, but follows the boat. The problem starts when it overtakes the boat, and the boat "fall backwards" into the trough of the underwater wave. The boat isn't dragged under, but its progress is halted Meanwhile, the surface looks smooth, and gives the passenger no clue as to what's happening underneath.
Image: Christopher Michel
[Source: Mattieu Mercier]