January 29, 1912, was a beautiful day in Antarctica. A group of British explorers, led by a 37-year-old Victor Campbell, were on a cheerful journey across what we now call the Nansen Ice Shelf and Priestley Glacier. It was a kind of summer sojourn around the continent: They would make the first maps of the area, then rendezvous with their ship, Terra Nova, six weeks later.

Campbell’s notes are brief on January 29. The terrain on which he and his team tottered around that day was at the foot of some glaciers and mountains, which loomed above the icy plain. The area even sounded different: “The noise of running water from a lot of streams sounded very odd after the usual Antarctic silence,” he wrote. “Occasionally an enormous boulder would come crashing down from the heights above, making jumps of 50 or 100 feet at a time.” His party set up camp that night on a bed of gentle gravel, then moved on.

But the Terra Nova did not reach them in February, or March, or ever. Early sea ice set in and blocked off the ship’s route. With winter bearing down, Campbell and his team took steps that later made them famous. They dug an ice cave on Inexpressible Island and made camp for the the winter. They remained in the cave for months— eating seal and penguin meat, burning blubber for warmth—all through the black night of Antarctic winter. Not until September 30 did they finally set off for the 200-mile march back to their basecamp.

For many years, that anguished winter was what made Campbell’s party famous, but for modern-day scientists of Antarctica, it’s a stray note in Campbell’s notebook that makes his journey worth remembering. Tucked away in the explorer’s 105-year-old journal is a passing mention of liquid water on Antarctica, a phenomenon that hydrologists are just now beginning to understand.

The first-ever hydrological survey of Antarctica has just been completed, and it found nearly 700 streams, ponds, and waterfalls, a sprawling and active meltwater drainage system never previously documented. The system appears to cover the entire continent, carrying water across both grounded ice and the floating ice shelves which surround its coast.

Its scale rivals anything found on the more temperate parts of the planet. Ponds can grow to be gargantuan—50 miles long—fed by streams carrying as much water as the Potomac or the Hudson. One stream system drains water from the heights of the Trans-Antarctic Mountains, more than 4,000 feet above sea level. Another ferries meltwater more than 75 miles, from glaciers on the land-surface of the continent to a large pond in the Ross Ice Shelf.

The survey was announced Wednesday in two papers in the journal Nature. It brought together decades of aerial and satellite photography of Antarctica with some of the earliest documented in-person observations.

“I think most polar scientists have considered water moving across the surface of Antarctica to be extremely rare. But we found a lot of it, over very large areas,” says Jonathan Kingslake, a glaciologist at Columbia University and one of the authors of the paper, in a statement. The survey also confirms that these ponds and rivers have existed in some form for decades.

The discovery fleshes out some major geographical features of Antarctica, the world’s least mapped land area. Most of the continent was not surveyed until the middle of 1957, when Soviet and Western scientists collaborated on the International Geophysical Year. (Imagine if we suddenly discovered several ponds the length of Rhode Island hidden in North America.)

“There was really so little known about the hydrology of Antarctica,” says Ian Willis, a glaciologist at the Scott Polar Research Institute at Cambridge University who was not connected to the paper. “About 10 years ago, I used to tell the students that there’s just starting to be an understanding of water and the Greenland ice sheet. The same is true of Antarctica today.”

The studies have an importance beyond the science of the world’s least-understood land area. They may require the recalculation of some of the longest-term estimates of sea-level rise, though it is unclear whether those projections will increase or decrease.

Contemporary models of the planet’s snow-and-ice system—the cryosphere—do not account for such an expansive meltwater network in Antarctica. Instead, they assumed that large meltwater ponds would rapidly melt and destroy ice shelves. In 2002, a 12,000-year-old ice shelf called Larsen-B disintegrated in less than six months after large ponds of meltwater formed on its surface. Researchers projected that future ice shelves would meet a similar fate.

But one of the papers raises questions about the breadth of that conclusion. The Nansen Ice Shelf—the same one first observed by Campbell—boasts a large drainage system that evacuates summertime meltwater off its surface and directly into the ocean. This system of streams and ponds terminates in a towering waterfall:

“The first time I saw that video, I went, “I just saw a picture of a waterfall in Antarctica,” says Robin Bell, a geophysicist at the Lamont-Doherty Earth Observatory at Columbia University. “I said, ‘Could I just take a picture with my iPhone so people don’t think I’m crazy?’ So for a long time, I was just walking around with this picture of this movie, because it was just so jaw-dropping.”

Most importantly for climate studies, the system—and the waterfall—seem to respond to slight changes in local temperature. After the coldest summers, the waterfall shuts down. And in the wake of warm summers, like the one from late 2014 into 2015, it flows at a thundering pace for almost a month. “At the low end of our estimate, it’s the Potomac,” Bell told me.

The existence of the Nansen system suggests that meltwater can sit on an ice shelf for more than a century without disintegrating its underlying structure. It’s still unclear whether that conclusion can be extended to other parts of Antarctica. (It certainly doesn’t seem to apply to Larsen B.) And there are outstanding questions about other streams and ponds on the continent, where the data record is scarcer.

The second paper offers the first published evidence of those streams and ponds. It began when Kingslake, the first author of the paper, spent hours trawling Google Earth for pictures of stream-looking objects.

But in an age where satellite imagery can seem ubiquitous, getting usable photography of the southernmost continent can prove surprisingly difficult. Landsat, the U.S. government’s scientific Earth-observation program, has been imaging almost the entire planet every two weeks since the mid-1970s, but it barely captures any images below the 82nd parallel south. What’s more, only some of this imagery can be used to map the meltwater system. The arcing paths of ponds and streams only blossom into their full size during the first months of the year, at the peak of the southern summer.

The same set of ponds appear on the Riiser-Larsen Ice Shelf across decades of satellite imagery. (Landsat / Kingslake, et al. / Nature)

Models won’t be able to take account of these dynamics yet because scientists still don’t understand the underlying physics. “The big question is: Why is it that water has been present on ice shelves for many years, for decades, and they’ve been relatively stable?” asked Willis. “I don’t think you can find anyone who can tell you an answer.”

Willis is, in fact, engaged in a project to measure how ice shelves respond to pooling water. He and two other researchers recently spent months in Antarctica, embedding GPS units in different aspects of an ice shelf in order to sense how it torques and flexes as meltwater moves across its surface. “If that water is simply evacuated, then it could be that those ice shelves are more stable than the models currently suggest,” he told me. “But it’s still pretty speculative.”

It’s also unclear how this research will ultimately feed sea-level models. Disintegrating ice shelves threaten to raise global oceans not because of the water they contain, but because they speed up the movement of the glaciers behind them, which are “grounded” on the Antarctic continent. If those ice floes speed up their drive to the sea, they could quickly juice sea levels.

But even if Antarctic ice shelves wind up looking more stable, estimates of sea-level rise before 2100 are unlikely to change. Most near-term sea-level rise will come from “valley glaciers” (ice on the other six continents), thermal expansion (the ocean’s tendency to enlarge as it absorbs heat), and the rapidly eroding ice sheets of Greenland.

Wednesday’s study shows how much there is still to be learned about the southernmost continent—and how much can still be extracted from what we already know. As part of her research, Bell later traveled to Cambridge to read the original Campbell party journals.

“People only remember these guys for having spent the winter in a cave, but in fact they did a month’s worth of marvelous mapping,” she told me. “I want to tell the story of these guys’ science and not just the terrible winter they had.”