The great white shark has the most fearsome reputation of all sharks. But it might not be the biggest of the predator sharks. That honor might go to the Pacific sleeper shark. The biggest one ever seen appeared to be about 23 feet long—longer than the biggest great white.

The Pacific sleeper is found mainly in cold waters around the rim of the northern Pacific Ocean. But some have been seen in warmer waters close to the equator.

The shark got its name because it was thought to spend most of its time near the bottom, waiting for prey to swim by—a “sleepy” sort of behavior. But at least one study found otherwise. The sharks were found to move up and down through the water column, from the bottom to near the surface. And some covered as much as three or four miles a day.

Pacific sleepers will eat just about anything. They prefer fish that dwell on the bottom, along with giant octopus. But their stomach contents also show other types of fish, snails, sea lions, and other prey. They might have hunted down some of them, and gobbled the already dead remains of others.

The shark hasn’t been studied that much. The largest one ever caught was about 14 feet long and weighed half a ton. But video cameras caught one that was estimated at 23 feet.

Pacific sleepers probably grow slowly and have a low reproduction rate. So they could be threatened by overfishing, mostly as bycatch—draining the population of what might be the largest of all predator sharks.

Currents at the bottom of the ocean can be just as fickle as wind currents at the surface. They can turn, speed up or slow down, and even reverse course. And they can change in just days or even hours.

That’s the conclusion of the most detailed study of sea-floor currents to date. Researchers anchored 34 instrument packages across a thousand-square-mile region off the coast of Mozambique, at the southeastern corner of Africa. The instruments monitored the currents for four years.

The study took place on the continental slope, at depths of up to a mile and a half. The slope is steep, and sharp canyons notch into it. Sediments tumble down the slope and through the canyons.

At the bottom of the slope, the currents generally flow from south to north. And in the canyons, they generally flow downhill. Speeds range from about a half to one-and-a-half miles per hour.

But researchers found a lot of variation. The speed changes, and so does the direction. Currents can even reverse direction—even in the canyons, where they sometimes flow uphill. Some of the changes are related to the tides or to passing storms or eddies. And others are related to the seasons, so they play out over days or weeks.

The researchers say a better understanding of sea-floor currents can tell them more about where ocean sediments come from. That can help them better understand changes in climate, the sources of pollution, and more—swirling along at the bottom of the sea.

In the spring of 1956, a doctor in the Japanese village of Minamata reported an outbreak of a troubling new disease. It was seen mainly among children, and it affected the central nervous system. The disease quickly spread, with hundreds of cases reported, then thousands. It took years for scientists to work out the cause: poisoning from industrial pollution in Minamata Bay—the first known case of a disease caused by polluted seawater.

A chemical factory was pumping huge amounts of wastewater into the bay. The water was laced with mercury. Some of it was methylmercury—an especially nasty form.

Microscopic organisms gobbled the stuff up, then were eaten by larger organisms. The amount of mercury built up to higher and higher levels with each link in the food chain. So the fish and shellfish eaten by people were filled with it. That triggered Minamata disease. Symptoms included numbness, problems with vision and hearing, trouble walking, and tremors. The disease killed hundreds, and may have afflicted millions. And its effects are still being felt.

The company dredged the bay to remove contaminated sediments. And the nations of the world crafted a treaty to reduce the amount of methylmercury in the environment. It calls for less mercury in products and manufacturing, fewer emissions of it from coal-fired power plants, and better storage and disposal.

Even so, mercury and other chemicals still cause problems as they work their way up the marine food chain.