The largest ice shelf in the Arctic has fractured, releasing all the water from the freshwater lake it dammed.
The Ward Hunt Ice Shelf is located on the north coast of Ellesmere Island in Canada's Nunavut territory.
The huge mass of floating ice, which has been in place for at least 3,000 years, is now in two major pieces.
The scientists who report the break-up in the journal Geophysical Research Letters (GRL) say it is further evidence of ongoing and accelerated climate change in the northern polar region.
The researchers - Warwick Vincent and Derek Mueller of Laval University in Quebec City, Canada; and Martin Jeffries of the University of Alaska Fairbanks, US - have been studying the shelf on site and through satellite radar imagery and helicopter over-flights.
The Ward Hunt Ice Shelf, which is 443 sq km (170 sq miles) in size, now has a major crack that runs right through it from north to south.
The scientists say the fracturing - which has been developing since the spring of 2000 - is the end result of a three-decade-long decline.
"We're now seeing some very extensive fractures in it that extend many kilometres horizontally across the ice-shelf; and they extend all the way through from the top to the bottom, many tens of metres through the ice shelf. And we've never seen fractures like this," Dr Jeffries told the BBC.
They warn that major free-floating ice islands could pose a danger to shipping and to drilling platforms in the Beaufort Sea.
The immediate consequence of the rupture has been the loss of almost all of the freshwater from the Northern Hemisphere's largest epishelf lake (a body of mostly freshwater trapped behind an ice shelf).
The freshwater lay in the 30km-long (20 miles) Disraeli Fiord.
At its deepest, the freshwater measured 43m (140ft), and sat atop 360m (1,200ft) of denser ocean water.
The loss of fresh and brackish water has changed the environment for the microscopic animals and algae living in the area.
"These are very rare and unusual ecosystems and they have been studied as possible analogues for life on a colder Earth and life on other planets," Dr Jeffries said.
"And if we are losing them, we are losing the opportunity to study life earlier in Earth history and elsewhere in the Solar System."
Scientists monitor continuously ice-shelf development in both the Arctic and the Antarctic.
In the southern polar region, recent times have witnessed some dramatic changes.
Last year, the 3,250-sq-km (1,250 sq miles) Larsen B Ice Shelf on the Antarctic Peninsula shattered over a period of a month into thousands of icebergs.
The peninsula is one of the three fastest-warming regions on Earth - temperatures have gone up 2.5 degrees in 50 years.
Mueller, Vincent, and Jeffries say their calculations suggest changes of a similar nature have been taking place in the Ellesmere Island area.
A century ago, the entire northern coast of the island was reported to be fringed with a continuous ice shelf. About 90% of that ice area had been lost by 1982, the scientists say.
The precise timing of the break-up of the remnant Ward Hunt Ice Shelf may have been influenced by freeze-thaw cycles, wind, and tides, they tell GRL.
Other factors may include changes in Arctic Ocean temperature, salinity, and flow patterns, they add.
"Computer models show quite convincingly that global climate change would be manifested first and amplified in the polar regions and in particular in the Arctic," Dr Jeffries said
"Our observations at Ward Hunt Ice Shelf fit in with a broader picture of Arctic change which fits in with our understanding of how the Arctic climate would respond to global change."