Sinking ice??

This weekend, I paddled on Oneida Lake on Saturday. I put in on the barge canal, and was able to paddle 1.5 miles out where I hit the edge of the iced over lake, it was solid, and at least 4-5" thick at that point. It appeared from my vantage point sitting in the kayak, that it was a solid, unbroken sheet for a very long ways, as in the whole rest of the lake. Went back Sunday morning, barely 12 hours later, and the lake was wide open! I find it hard to believe that the entire lake just melted over night. And a gentlemen in the bar that we stopped at after our paddle, said it didn't melt, it sank! Is that possible? I always thought ice floated on water. But it seems like a more likely explanation than 5" of ice melted totally away in 12 hours on a 45-50 degree night. Any one have any ideas on this phenomenon?