That part of TN is underlaid by karst (fractured limestone) under a relatively thin layer of soil. Limestone is, of course, water soluble (hence caves). A sinkhole is normally just a collapsed cavern. Madison county is also underlaid (for the most part) by karst, which is why the national headquarters of the Speleological Society is here (on Cave Avenue). In fact, the annual concert held in the Three Caves at the top of Fagan Springs Dr. is this week, I believe. As you go north in Madison County, the soil layer thins until it's impossible even to think about driving an 8' ground rod at the TN line. Karst, though water soluble is hard, really hard. On Monte Sano, being part of the Cumberland Plateau, we're underlaid by sandstone, with varying depths of soil. On my three acres, I have one area, by my shop, where I'm supposed to have 8' of soil. I spent some time this afternoon trying to drive an 8' ground rod for my generator. No luck. 6.5' was it, and I was happy.
All this to say that a sinkhole in that part of TN is far from surprising. OTOH, we should be careful about laughing prematurely. Our stadium is underlaid by a large aquifer. A collapse is far from impossible...
