There’s a big hole in the Indian Ocean. It’s nothing you can actually see. And the ocean itself isn’t especially deep. Instead, it’s a hole in Earth’s gravitational field—the weakest pull across the entire planet.

The “hole” was discovered in 1948. It’s centered about 750 miles off the southwestern coast of India. It covers more than a million square miles—more than a third the area of the Lower 48 states. Gravity there is so weak that surrounding regions of the ocean pull water away from it. As a result, sea level above the hole is about 350 feet lower than the global average.

In 2023, using computer models of the motions of the plates that make up Earth’s crust, scientists suggested the hole may be the remnant of another ocean—the Tethys Ocean. It vanished tens of millions of years ago.

The ocean was wedged between two “super”-continents—slabs that held most of the world’s total land area. But the motions of the plates pulled apart one of the continents. That pushed the plate that held the Tethys Ocean deep into the mantle—the layer below the crust.

The ocean floor reached its deepest point below the surface about 20 million years ago. It pushed away dense blobs of rock, allowing lighter rock to bubble up from below. The lighter rock exerts a weaker pull than the rocks around it.

Scientists still need to confirm that scenario—a possible explanation for a giant “hole” at the bottom of the Indian Ocean.

For an oyster, gender is more than a matter of genetics—it’s also about the environment. Water temperature, salinity, pollution, and other factors determine whether an oyster will be male or female. And a recent study added something new to the list: acidity.

The oceans are becoming more acidic as they absorb carbon dioxide from the air. Over the past couple of centuries, they’ve taken up about a third of all the CO2 added to the atmosphere through the burning of fossil fuels. Today, the concentration of CO2 in the oceans is at its highest in 800,000 years. By the end of the century, it could be at its highest in 20 million years.

The more-acidic waters make it harder for oysters and other creatures to make their shells. And researchers looked at the impact on the sex of oysters. They gathered oysters from the wild and from hatcheries—both in China—and put them in tanks with different levels of acidity. The oysters in the more-acidic water spawned about three times more females than males.

The scientists then placed the new generation in two locations in the wild, with different levels of acidity. Both groups spawned more females than males, but the ratio was higher in the more-acidic waters.

Researchers conducted lab studies to understand how this happens. They found that the higher acidity turned on female-producing genes, and turned off the male-producing genes.

So oysters face one more threat from the world’s changing oceans.

The great white shark probably is the most feared of all ocean animals. It gained that scary reputation 50 years ago thanks to a blockbuster movie: Jaws. The movie premiered on June 20th, 1975, and quickly became the all-time box office champion. It scared a lot of people out of the water—and set off a frenzy of shark killings. People killed thousands of them, and even competed in shark-hunting tournaments.

Great whites can be fearsome. They’re responsible for more human deaths than any other species of shark. In fact, a series of attacks along the New Jersey coast in 1916 was one of the inspirations for the novel upon which the movie was based.

But the sharks aren’t nearly as nasty as portrayed on screen. The movie version was much bigger than any real great white ever seen. And it was stronger and more tenacious.

Biologists say that great whites don’t pursue humans. Instead, a shark may attack because it confuses a person or surfboard with a seal, one of its favorite meals.

Great whites don’t have especially good eyesight, and they probably are color blind. So they look for dark silhouettes on the surface contrasted with the bright sky above. And scientists recently suggested a way to use that trait to keep the sharks from attacking. Researchers towed seal decoys behind a boat in shark-infested waters. And they found that adding stripes of bright lights to the underside of a decoy kept the sharks at bay—making it safer to get back in the water.

When tropical storm systems barrel across the Atlantic Ocean toward North America, they often take aim at the Lesser Antilles—an arc of small islands that marks the eastern boundary of the Caribbean Sea. In fact, the islands often are the first thing forecasters talk about when a tropical system heads our way.

The Lesser Antilles arc from the U.S. Virgin Islands, near Puerto Rico, all the way down to South America. They comprise three separate groups: the Leeward Islands, the Windward Islands, and the Leeward Antilles. The Windwards extend farthest into the Atlantic. They were the first stop for European sailing ships, which followed the prevailing winds to the Americas.

Most of the islands are volcanic. They formed as two of the plates that make up Earth’s crust plunged below the Caribbean Plate. As the rock descended, it melted. Some of the molten rock then forced its way upward, building the islands.

Earthquakes rock some of the islands. And some of the volcanoes that built the islands are still active. A massive eruption that began in 1995, for example, destroyed the capital of the island of Montserrat, and forced most of the population to leave the island.

Despite the volcanoes and hurricanes, many of the individual islands are popular tourist sites. Places like the Virgin Islands, Aruba, and Martinique offer tropical beaches, coral reefs, rugged mountains, and other natural attractions—at the edge of the Caribbean Sea.

For some coral, home isn’t where the heart is—it’s where the sounds are. A recent study showed that one type of coral prefers to settle on reefs that sound healthy—even if they’re not.

Young corals, known as larvae, float or swim through the water for a while. When they find a good spot, they drop to the bottom and grab hold. They use several cues to find the best locations, including the lighting and chemistry. And according to the study, one of those cues might be sounds.

Biologists recorded the sounds of both healthy and un-healthy reefs in the U.S. Virgin Islands. Healthy reefs are noisy. They feature the clicks of snapping shrimp and the grunts and groans of fish. Unhealthy reefs are much quieter.

The scientists placed small groups of larvae in special containers on three reefs. One reef was healthy, with a good amount of coral. The others had less coral and much more algae, which can kill a reef.

Researchers played the sounds of a healthy reef to the larvae on one of the damaged reefs. On the other two reefs, the larvae had only the natural sound of the environment.

On average, the larvae that were played the healthy sounds were 1.7 times more likely to settle on the reef than those at the other reefs—even the healthy one. So playing a nice lullaby into damaged reefs might lure new generations of coral in the years ahead.

Our thanks to T. Aran Mooney of the Woods Hole Oceanographic Institution for the reef sounds.

The oceans are losing their memory. That could make it harder to forecast everything from monsoons to blizzards.

Ocean “memory” is maintained in the top layer, called the mixed layer. Winds push warm surface water downward, where it mixes with water at greater depths. This layer is typically about 150 feet thick. And overall, it maintains a fairly constant temperature. When the temperature changes as the result of some major event, it can take 10 to 20 years for the change to dissipate. In other words, the ocean maintains the “memory” of what happened to it for that long.

As the air warms up, though, ocean memory may be changing. Researchers recently simulated changing ocean conditions with computer models. Their work suggested the memory span may be getting shorter, mainly because water at the surface is getting warmer and less dense, so it doesn’t sink as easily. As a result, the mixed layer gets thinner, so its temperature can change more quickly. The quicker the change, the shorter the memory. As one researcher described it, the ocean develops amnesia.

Ocean memory is an important factor in forecasting conditions in both the oceans and the atmosphere. Scientists use it to predict monsoon seasons, heatwaves, exceptionally wet summers and cold winters, and other major climate events. A shorter ocean memory could cut months from the lead time of these forecasts—making it harder to remember whether to take an umbrella or sunscreen on your next outing.

Otto the octopus didn’t appreciate the spotlight shining on his laboratory tank at night. So he turned it off. In fact, he turned off the lights in the whole lab.

Octopuses are smart and curious. They use tools and learn from watching other octopuses. They can solve mazes, open the lids of glass jars, and build dens.

In the lab, they learn to tell people apart. They’ve been known to take a disliking to some of the lab workers, squirting jets of water at them when they walk by. And they can show a clear preference for the people who feed them.

Octopuses also play. In one study, scientists put some small, sealed plastic bottles in octopus tanks. Some of the critters fired jets of water at the bottles, bouncing them off the walls. Others fired a bottle toward the tank’s inlet valve, so the bottle came back to them—like octopus ping-pong.

And that brings us back to Otto. Scientists in Germany were studying the behavior of Otto and several others. Otto was the most active. He damaged the glass walls of his tank by throwing rocks at them, and he sometimes rearranged the stuff in the tank.

A 2,000-watt spotlight shined on the tank at night. But several times, the light shorted out—and so did the rest of the lab. Scientists then spent the night in the lab to figure out what was happening. Otto was climbing to the rim of the tank and squirting water at the light. That turned out the lights—perhaps allowing Otto to get a good night’s sleep.

A rare species of sponge found in the Red Sea and Indian Ocean has a rare distinction: It has levels of a metal that are thousands of times higher than ever seen in any other organism. And most of that metal is stored away by a type of bacteria that lives inside the sponge.

Sponges are filter feeders—they draw in water, filter out food and other solid bits, then shoot the water and solid particles back into the sea. Some contaminants can lodge inside them. Some studies have found high levels of arsenic, barium, and other toxic substances.

A more recent study looked at the species called Theonella conica. It’s found down the eastern coast of Africa. Up to 40 percent of its body weight consists of bacteria and other microscopic organisms, many of which have a symbiotic relationship with the sponge.

Over two decades, researchers collected specimens from Zanzibar, off the southeastern African coast, and the Gulf of Aqaba, at the northern tip of the Red Sea.

When they analyzed the sponges, the scientists found extremely high levels of molybdenum. The metal is important for the metabolism of people and other animals, but only at low levels. At high levels it’s toxic. So the metal may help protect the sponges from predators.

The element was concentrated in one of the species of bacteria inside the sponge. It changes the material into a harmless mineral, which is flushed back into the water—keeping the sponge safe from both predators and the toxic element.