In my case, it definitely was from a small, sharp rock in heel of the sole of my paddling shoe, as I discovered later. Like Magooch, I normally do not allow sand or gravel to get into my kaykak, so I paddled for years with no observable wear. However, I wore the hole in the inner layer of my carbon hull during just one four-hour paddle when I was unaware of the rock in my sole. I immediately installed the HeelEasy material and have had no further problems.