This is when warm ocean water likely first got under Pine Island Glacier (PIG) to loosen the secure footing it had enjoyed up until that point.
Researchers figured out the timing by dating the sediments beneath the PIG.
It puts the glacier’s current changes in their proper historical context, the scientists tell Nature magazine.
These changes can now be regarded as unprecedented in thousands of years.
Not only is the glacier going backwards, it is also thinning fast - losing more than 2m in elevation every year.
Other field studies and computer models suggest a runaway collapse might even be possible.
"This glacier used to be pinned to a ridge and once it moved away from that ridge, it started to retreat rapidly; and without other pinning points it could continue to retreat rapidly inland, contributing significantly to global sea level," Dr James Smith from the British Antarctic Survey told BBC News.