I've run across something, not quite like that, but similar.
I used to ride and trout fish remote streams in the Michigan's upper penninsula and one hot August day a friend and I were riding east of Grand Marais and stopped for a break at a place called Log Slide Overlook. It's a place where there are fairly high dunes going down to Lake Superior. There used to be a log slide there in the logging days where logs were slid down a ramp from the top of the dune down into the lake.
Anyway, we were hot, sweaty and dusty so we stripped to our underwear and ran down the dune to take a dip in Lake Superior. About 1/3 of the way down the dune we both fell right through the dune and landed in a hollowed out snow drift left over from the previous winter that had been covered and insulated with sand and hadn't melted completely. The rush of falling through something unknown and landing in deep snow was completely shocking and scary. We crawled out of the void and went on to the lake for an almost equally cold and shocking swim. Lake Superior never gets very warm.