For a brief moment last year, Angel Falls — looming 979 meters (3,212 feet) over Canaima National Park in Venezuela — was likely dethroned as the world's tallest waterfall. The usurper, according to researchers from the University of Cambridge, was a massive fracture that opened underneath a meltwater surface lake thousands of miles away on the Greenland ice sheet. About 5 million cubic meters (1.3 billion gallons) of water — roughly equivalent to 2,000 Olympic-sized swimming pools — plunged straight down to the bedrock below, reducing the lake's area to a third of its original size in as little as five hours.

Drone imagery captured the lake drainage in unprecedented detail. The image on the left was taken at ~13:00. About 5 hours later, the lake started to drain, and by 02:00 (right), it had lost two thirds of its volume. The main fracture responsible for the drainage can be seen near the bottom of the image. (Photo: Sarah Collins/Cambridge University)

It's common for meltwater lakes residing on ice sheets to experience catastrophic fractures and quickly drain through cavities known as moulins, but to date, scientists have relied on satellite data to document the process. This time was different. While on site conducting research, the University of Cambridge team was able to record the swift drainage in real time using specially designed drones.

Using sensors in the ice and multiple drone flights, the researchers were able to track the flow of water as it drained through the fracture and under the surface. In a paper published in Proceedings of the National Academy of Sciences, they explain how the massive influx of surface water caused "the ice flow to accelerate from a speed of two meters per day to more than five meters per day as surface water was transferred to the bed, which in turn lifted the ice sheet by half a meter (1.5 feet)."

Moulins like this one, often plunging more than 3,000 feet to the bedrock below, are responsible for quickly draining massive surface lakes on the ice sheet. (Photo: Sarah Collins/Cambridge University)

In partnership with researchers from Aberystwyth and Lancaster universities in the United Kingdom, the team was able to reconstruct the data into 3D models to show how meltwater drainage influences the formation of new fractures and the expansion of dormant ones. It also supports a computer model proposed by the Cambridge scientists that such lake drainages occur in a dramatic chain reaction.

"It's possible we've under-estimated the effects of these glaciers on the overall instability of the Greenland Ice Sheet," co-first author Tom Chudley, a Ph.D. student at the University of Cambridge and the team's drone pilot, said in a statement. "It's a rare thing to actually observe these fast-draining lakes — we were lucky to be in the right place at the right time."

Researchers flew drones over the lake as it was draining, building 3D models of the ice sheet surface as well as capturing spectacular images of waterfalls entering the depths of the ice sheet. (Photo: Tom Chudley/Cambridge University)

As the Greenland ice sheet is the single largest contributor to global sea-level rise, the research team will continue studying how these drainage events might hasten its decline as the climate continues to warm. Their next step is to use drilling equipment to observe first-hand how mass quantities of surface meltwater is accommodated in the subglacial drainage system.

"The Greenland ice sheet has really dramatically changed over the last 30 years," Chudley told Scientific American. "And we need to understand the processes that are going on."