Here in Iowa this has been a brutal winter with temps for extended times 30 below with windchill 50 below and plenty snow.

+ Doughboy 16x32 above ground about 1.5 years old with the best liner they had.
+ Always removed snow off of winter cover with roof rake if more than 4 inches accumulated,maybe more often than need.Last year it was cold but not like this year.
+ I always lowered water level to 6" below return.

Just out of chance, today I decided to look at the cover and cond. of cover since the temps have been in the upper 30's /40's.
I freaked out when I noticed the pool cover down further than before in the center,tapped around the wall and felt like it was empty,able to push in on the wall.
I loosened the cable,pulled away some of the cover to look inside pool and noticed the frozen water almost like a ball and sunk down in the middle, kind of like a doughnut.
Totally dis-configured frozen water.

Temps have been near 50 off and on for a day or 2 before re freezing for the last 2 weeks.

Just returned from Florida after a week, so I dont know for sure what may have happened.
With the center down further than were I originally set the water level to, it is stretching the cover on the ends and caused some separating of the material which cause an open hole. Last year the water froze flat accross with no issues.

Any ideas?
thanks