Is this visible now because there has been more melting of the ice or did people just not recognize it before?
Used to be there were no craters on the Earth, because of erosion. IOW, no one bothered to look for them, and when they were pointed out or obvious as hell, they were characterized either as erosional features (like the Carolina Bays still are by some people) or as “cryptovolcanic” (as the Barringer Meteor Crater was). Nearly 200 have been identified, mostly large like the Manicougan structure.