Turnpike officials tell Action News the pothole, measuring about 4-feet wide by 4-feet across, opened up around 7:30 a.m. Tuesday.
The hole was located in a bridge deck where the turnpike crosses over North Wales Road at mile marker 24.8 southbound. That's about 4 miles north of the Mid-County tolls.
A contractor was on the scene making repairs. The left lane was closed, and traffic was getting by in a single lane past the repair site.
The backup was reported to be 8 to 10 miles long.
Motorists have been advised to avoid the area.
Turnpike officials say they hope to have both lanes back open by the Tuesday afternoon rush hour.