These "blue holes" are all over southern NJ and are the result of sand mining that dredged the sand out for glass making, concrete making, etc. There are still some active ones, but a lot are abandoned. People drown in these all the time. The shorelines are not stable or packed down and drop off very suddenly. The water temperatures fluctuate and get very cold in the middle which causes swimmers to cramp up. There is old abandoned equipment and cars in some of these. Some of them are 100' deep or more.