The immense and forbidding Southern Ocean is famous for howling winds and strange waves that have tested mariners for centuries.

But its true strength lies beneath the waves.

The ocean’s dominant feature, extending up to two miles deep and as much as 1.200 miles wide, is the Antarctic Circumpolar (极地附近的) Current, by far the largest current in the world.

It is the world’s climate engine, and it has kept the world from warming even more by drawing deep water from the Atlantic, Pacific and Indian oceans, much of which has been in the deep ocean for hundreds of years, and pulling it to the surface. There, it exchanges heat and carbon dioxide with the atmosphere before being pushed again on its endless round trip.

Without this action, which scientists call upwelling, the world would be even hotter than it has become as a result of human-caused emissions of carbon dioxide and other heat-trapping gases. “From no perspective is there any place more important than the Southern Ocean,” said Joellen L. Russell, an oceanographer at the University of Arizona. “There’s nothing like it on Planet Earth.”

For centuries this ocean was largely unknown, and its conditions were so extreme that only a relative handful of sailors went to its waters with lots of icebergs.

What fragmentary scientific knowledge was available came from measurements taken by explorers, naval ships, the occasional research expeditions or whaling ships.

But more recently, a new generation of floating, autonomous probes (探测仪) that can collect temperature, density and other data for years—diving deep underwater, and even exploring beneath the Antarctic sea ice, before rising to the surface to phone home—has enabled scientists to learn much more. They have discovered that global warming is affecting the Antarctic current in complex ways, and these shifts could complicate the ability to fight climate change in the future.

The World’s Most Amazing Caves

Humans have been drawn to caves for thousands of years. Here are four of the world’s biggest caves.

Son Doong Cave, Vietnam

Son Doong Cave in the remote(偏远的)forests of Vietnam holds the title of the largest cave in the world. Son Doong is a spreading network of tunnels and caves, the largest of which could contain an entire New York City block with 40-story buildings. Parts of the cave’s top have fallen down，creating vast skylights that allow life to grow to a height of hundreds of feet beneath the forest floor.

Mammoth Cave, Kentucky, the United States

Mammoth Cave in Kentucky is the longest cave system in the world, with 426 miles explored—and scientists guess another 600 miles have yet to be discovered based on measurements of air movement within the caves. Mammoth Cave has drawn visitors since the 1800s, and 10 miles of underground passages are accessible to tourists today.

Sac Actun. Mexico

The longest underwater cave system in the world, Sac Actun winds(蜿蜒)more than 215 miles under Mexico’s Yucatan Peninsula. Adventurers can take a drop in the Sac Actun system through more than 248 cenotes(天然井)—the natural swimming pools that form when sinkholes connect the surface to the underwater caves.

Veryovkina Cave, Abkhazia, Georgia

Veryovkina Cave is the deepest cave in the world, spreading 1.4 miles beneath the surface. It takes professionals a week to make a round journey to Veryovkina Cave, They have to brave waterfalls, tight presses and sharp drops. But the payoff is excellent—Veryovkina

Cave’s end is a beautiful blue lake about 50 feet long and 25 feet wide surrounded by black black rocks.

NASA scientist Josh Willie flew over Greenland this week, and gazed at a widespread polar world of melted ice and dark pools of water.

In mid-August, a powerful heat wave melted large areas of the Greenland ice sheet, which is three times the size of Texas. It's a vivid sign of changing times, and climates. In recent decades, Arctic scientists have observed record-breaking melt events in Greenland, which result in water pouring into the sea - and contribute to sea level rise. So far in 2021, the island's melted area (8.2 million square miles) is way above the 1981 to 2010 average-to-date, by some 1 million square miles.

"What's important to know is that all the big melt years have happened in the last two decades or so," Willis, who researches ice sheets and oceans at NASA's Jet Propulsion Laboratory, told Mashable between Greenland flights. "That's because the melt in Greenland is getting more extreme with every decade due to human interference with the climate."

Arctic scientists have found that, over the last couple decades, Greenland is melting faster than it has in at least 350 years. The ice sheet is shrinking.

As the climate warms, and heat waves become increasingly extreme, major melting events are even happening at the island's typically freezing summit. This summer, a couple of serious heat waves hit Greenland, noted Ted Scambos, an ice sheet expert at the University of Boulder Colorado who has no involvement with the NASA mission. It even rained on Greenland, at 10,551 feet. Scientists had never observed rain there before.

Before 1995, centuries would go by between major melt events at the summit, explained Scambos, referencing evidence from cores of ancient Greenland ice. Now, it's occurred a handful of times in recent decades, he said.