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.