Yesterday, the Nathaniel B. Palmer left Antarctica behind and made the turn toward home. The last science experiments were completed, and the ship headed north, toward Punta Arenas, Chile, where our two-month journey will end. Scientists on board are packing up equipment and writing rough drafts of papers based on discoveries they made during our adventure into uncharted waters around Thwaites glacier. But an almost existential question looms above it all: Did we just witness what amounts to a climate catastrophe playing out in real time?

On March 3rd, Bastien Queste, an oceanographer at the University of East Anglia who is a key member of the science team aboard the ship, got a WhatsApp message from a colleague back in the UK. She had sent him a satellite image of Thwaites glacier and the surrounding region in West Antarctica. At the time, we had just completed our own close encounter with the awesome craggy blue glacier and were only a few miles away, mapping the seabed in front of the glacier with the ship’s sonar device.

On this trip, satellite images have been indispensable in helping scientists track the ever-changing ice in the regions we’ve been exploring. But the map Queste received that morning was different. He noticed dark cracks in parts of the ice shelf, which floats out over the sea like a huge fingernail from the glacier itself. They had not been there before. The ice shelf was clearly starting to break up. Queste’s first thought: “Oh, shit.”

Queste knew as well as anyone, the whole point of this research trip is to better understand the risk of collapse of Thwaites glacier, one of the most consequential tipping points in the Earth’s climate system. It’s not just that Thwaites is big, although it is (imagine a glacier the size of Florida). But because of how the glacier terminates in deep water, as well as the reverse slope of the ground beneath it, Thwaites is vulnerable to particularly rapid collapse. Even more troubling, Thwaites is like the cork in the wine bottle for the rest of the West Antarctica ice sheet. If Thwaites were to fall apart, scientists fear the entire ice sheet could begin to collapse, eventually raising sea levels more than 10 feet.

That’s what Queste’s “oh, shit” was about. It was non-scientific-but-very-human-shorthand for, “Is Thwaites falling apart in real time, right before our eyes?”

Queste showed the image to Rob Larter, the chief scientist on the Palmer. Larter was not entirely surprised by what he saw. A few days earlier, as we cruised along the front of Thwaites, Larter had remarked on how chaotic and jumbled the ice shelf looked. “I thought something like this might happen because of how broken up the ice on the shelf appeared,” he says.

Over the next few days, Queste and Larter — as well as nearly every other scientist and student on the ship — watched the disintegration of the Thwaites ice shelf. It was a spooky sensation, looking at the satellite images then looking out the window as a parade of icebergs floated right by us on their way out to sea. In a matter of 48 hours or so, a mélange of ice about 25 miles wide and 15 miles deep cracked up and scattered into the sea. As Queste says, “A part of the glacier that is as big as the city I live in — it was just gone.”

For scientists both on and off the ship, the big question is, was the blowout a sign that Thwaites is collapsing before our eyes, or was it a more or less ordinary event in the lifecycle of a big glacier? These are not easy questions to answer. Glaciers are practically alive, in flux all the time, exquisitely sensitive to small changes in atmospheric and ocean temperatures. Sometimes changes that look dramatic to non-scientists, like the breakup of the Larsen B ice shelf in Antarctica a few years ago, have an inconsequential impact on sea level rise (unlike the Thwaites ice shelf, the Larsen B is not holding back a massive city-drowning glacier).

And it’s important to point out that the Thwaites blowout is not the same thing as what scientists typically call a “calving event,” which you often see in movies and documentaries, where big slabs of ice fall off glaciers into the sea. What we witnessed was the sudden disintegration of an ice shelf, which is a very different thing. Unlike the calving of land-based ice into the sea, the break-up of an ice shelf does not itself contribute to sea level rise, because the ice is already floating — just as when the ice in your whiskey melts, the level of whiskey in your drink doesn’t rise.

Nevertheless, ice shelves are important. They buttress the glacier itself, providing stability and in effect holding it back from slipping faster into the sea. The ice shelf that blew out at Thwaites was particularly messy and chaotic — it’s a bunch of bergs glued together with seasonal sea ice rather than a solid shelf. So maybe it wasn’t doing much to stabilize Thwaites, and the blowout wasn’t a big deal.

But given the larger fragility of Thwaites, and the consequences of a sudden collapse, any dramatic change in the structure of the glacier is hardly an encouraging sign.

When it comes to melting glaciers and sea level rise, climate scientists have traditionally been far more concerned about Greenland than Antarctica. In our warming world, Antarctica was viewed as a stable place: very big, very cold, very distant.

Then, in the early 1990s, improved satellite observations proved those assumptions were wrong. Of special concern was West Antarctica, which is particularly vulnerable to warm Circumpolar Deep Water attacking the glaciers from below. A recent paper co-authored by scientists from NASA’s Jet Propulsion Laboratory in California noted that the main trunk of Thwaites accelerated 33 percent between 2006 and 2013 — it’s now sliding into the sea at a rate of about two miles per year. In addition, parts of the glacier are thinning by as much as 13 feet each year.

So what does all this mean? Nobody can say for sure. “I’m holding my breath to see what happens next,” says Larter. “The blowout could be the start of a new phase of the evolution of Thwaites glacier. But I’m wary, because sometimes you see things that you think are going to be the start of something big, and then things settle down. I think it’s too early to say which way this is going to go.”

In an email, Eric Rignot, a senior scientist at NASA’s Jet Propulsion Laboratory who has co-authored recent papers suggesting the collapse of Thwaites is already past the point of no return, told me that he wouldn’t view the blowout as particularly alarming unless he saw retreat of the front of the glacier itself during the process (which, in the most recent satellite images, he hasn’t). But Rignot ended his email to me with an important note: “This sort of event is a good reminder that changes can happen fast in these environments, even though it may seem that nothing much is happening when you are staring at the glacier from a ship deck, right?”

Richard Alley, a highly respected glaciologist at Penn State, had a more nuanced view of it all. Alley (who, like Rignot, is not on the ship) pointed out that because the ice shelf that blew out was already pretty chaotic, it was likely not providing much stability to the glacier. “So its loss is not a huge issue for the still-grounded ice,” Alley emailed. “But the chaotic ice was still doing something.” He compared Thwaites to glaciers in Greenland, where the blowout of similar mélanges are often followed by calving of ice from the glacier itself, which is far more troubling. Alley also pointed out that the loss of ice shelves leaves glaciers vulnerable to stress from what he called “remote forcing” — storms across the Pacific, or tsunamis from an earthquake. “To stretch the analogy a little bit,” Alley said, “if Thwaites were a car, you could say that it has lost part of its bumper. And, while that’s not hugely important, it is part of a pattern that is pointing toward larger changes to come.”

This is, in short, what makes climate change so alarming, and so unlike other threats that humans have faced. By loading the atmosphere with carbon, we are messing with a system that even the best scientists in the world don’t fully understand. Individual events are hard to interpret in real time. “In the history of human civilization, we’ve never seen the rapid collapse of a glacier like Thwaites,” Larter points out. “So we don’t know how exactly it starts, or what it looks like while it’s happening.”

But in the long run, the arc of uncertainty bends toward catastrophe. It may be that this blowout at Thwaites was driven by wind or a shift in ocean temperature that, in the big picture, means little. Or it may be further evidence that the collapse of Thwaites is already underway, and it’s only a matter of time — perhaps even during the lifetimes of kids alive today — before virtually every coastal city from Miami to Jakarta is under six, seven, eight or more feet of water.

If that’s the case, then big parts of the world we live in today may already be doomed. We just don’t know yet.