Sounds like a blowout to me. There was probably a thin layer of soil covering a rock crevice or soil pipe. When the pressure became great enough the soil 'creeped' and caved in. With such a sudden occurence and obviously tremendous flow (1 ft/day) I can't imagine what else it could be. What I would do is this. Choose a nice calm day with little wind. Buy several bales of good wheat straw and scatter it evenly over the pond surface. Then turn on your pump and pump water into the pond just as fast as you can. (may require more temporary pumps to generate sufficient flow) Then watch the straw until you see some sort of pattern develop possibly a slowly rotating whirlpool or straw accumulating at a particular point near the shore. You'll probably have to drain the pond to fix it. A sudden leak like that should leave a pretty good mark on the basin.