The liner of my pond has a couple of areas where the liner bulges up about 6 inches. The diameter of the bulges is about 12-18 inches. When I accidentally step on these bulging areas, the liner goes down but then comes right back up again. I have never had any leaks but I worry about this. What prevents these bulging areas from leaking? This pond was built nearly 50 years ago and I have no idea of the materials used. The size of the pond is about 60,000 gallons, with the area being about 100' long by 50' wide. Draining it to inspect the liner is not an option.