One of the buzziest technologies in modern science may be running right under your feet. Fiber optic cables bring you the internet as data-rich pulses of light, but they also detect signals from the surrounding environment: Researchers can analyze the light that’s scattered when a volcanic eruption or tsunami jostles the wiring. Known as distributed acoustic sensing, or DAS, the technique is so sensitive that it can track your footsteps as you walk over a cable, and may one day even warn you of an impending earthquake.

Now, researchers have laid a fiber optic cable on the seafloor near a glacier in Greenland, revealing in unprecedented detail what happens during a calving event, when chunks of ice drop into the ocean. That, in turn, could help solve a long-standing conundrum and better understand the hidden processes driving the rapid deterioration of the island’s ice sheet, which would add 23 feet to sea levels if it disappeared.

Even before humans started changing the climate, Greenland’s glaciers were calving naturally. The island is covered in glaciers that slowly flow toward the ocean, breaking into icebergs that float out to sea. When temperatures were lower, the ice sheet was also readily regenerating as snow fell. 

As temperatures have climbed, though, more melting is creating more meltwater, which flows underneath glaciers, lifting and lubricating them. “It can actually affect how fast the ice flows,” said Michalea King, a senior research scientist at the University of Washington’s Polar Science Center, who wasn’t involved in the new research. “So not only do you have the loss of mass from the melt directly at the surface, but then you’re also impacting how rapidly these big conveyor belts of ice — these big outlet glaciers — are flowing.”

Accordingly, Greenland now sheds much more ice than it regenerates. “It’s like you’re spending more out of your checking account, and so your account balance has been going down for a couple decades,” said Paul Bierman, a geoscientist at the University of Vermont and author of When the Ice Is Gone: What a Greenland Ice Core Reveals About Earth’s Tumultuous History and Perilous Future. (Bierman wasn’t involved in the new research.) “This paper is a big advance in that it gives us some of the process details in places where we really haven’t had it before.” 

The challenge is that models majorly underestimate the amount of ice melting where Greenland’s glaciers touch the sea, suggesting that they’re not accounting for a process that is amplifying that net loss. That’s not due to a lack of effort from glaciologists — it’s just extremely dangerous to get up close to massive chunks of falling ice to collect data. 

Taking a different tack in a fjord in south Greenland, researchers strung 6 miles of cable parallel to a glacier’s “calving front.” Whenever the glacier fractured, or dropped ice into the water, it “plucked” the cable, like a guitarist plucking a string. These vibrations scattered light in the fiber optics back to two “interrogator” devices, powered by solar panels and batteries, on land. One of these handled the DAS data, or the acoustics propagating through the water, while the other determined temperature changes in the fjord. “If you fracture wood, you see the fracture propagating, but you also hear it,” said Dominik Gräff, an environmental scientist at University of Washington and lead author of a new paper describing the work in the journal Nature. “That’s exactly what DAS does.”

These glacial fractures look distinct in the DAS data from a more catastrophic loss of ice into the fjord, the calving from the ice front. “These ice blocks can be as big as a stadium,” Gräff said. “When they plunge, they excite these waves.”

If you’ve seen video of a calving event, you know how dramatic that excitation can be, as a wall of water rushes away from the ice. (That’s technically classified as a tsunami, though a much smaller one than those that move across whole oceans after earthquakes.) But the DAS system also picked up a hidden movement of water beneath the surface, as waves — some as tall as skyscrapers — pulsed across the seafloor cable, raising and lowering the interface between cold surface waters and warm deep waters.

Typically, warmer, saltier water sinks to the bottom because it is denser, while colder, fresher water from glacial melt sits at the surface. The latter also forms a sort of insulating layer at the edge of the glacier, preventing more melting. But the fiber optic cable showed that as an iceberg dropped into the fjord, it stirred those warmer waters to the surface and disturbed the insulating layer, thus encouraging more melting of the glacier. And as the iceberg drifted away from the glacier, it stirred still more water, like a boat creating its own wake, but invisible under the surface.

This could be the missing piece of that scientific puzzle, as models aren’t representing this widescale stirring, which could be encouraging more calving, which produces more stirring, which encourages more calving. “Maybe this study is the key to why, in practice, in real life, we have much higher melt rates than what we would expect,” said Mathieu Morlighem, a glaciologist at Dartmouth College who wasn’t involved in the research. “They are able to capture a lot of the physics that we didn’t even know was happening.”

In contrast to scientists boating around a calving front, fiber optic cables cheaply, safely, and passively collect reams of data. These researchers were only able to operate their cable for three weeks, but they plan to do further studies that use readings from much longer timescales, monitoring how calving changes throughout the year. If they’re able to deploy more cables near Greenland’s coastal cities, they might even be able to design an early-warning system for ice-induced tsunamis, like other scientists are trying to do with DAS for earthquakes.

Now it’s a race against time to better understand Greenland’s ice as it falls into deeper peril, as calving begets more calving. “That’s the kind of thing that scares geoscientists like me,” Bierman said. “That if you have these reinforcing feedback loops, and you start down a path of losing ice from Greenland, that could accelerate the rate of that loss.”