This final collapse of the surface might take anything from a few minutes to several hours.

BBC: Who, What, Why: How are sinkholes formed?

"It can be very difficult to predict collapse because there is very little surface evidence of the features, " says Dr Vanessa Banks, an expert in shallow geohazards and risk at the British Geological Survey.

BBC: Who, What, Why: How are sinkholes formed?

As well as this reduction in area, scientists believe that, hidden beneath the surface, the ice is growing ever thinner, setting up the Arctic for another sudden, catastrophic collapse.

ECONOMIST: An imminent answer to an Arctic riddle