This is why I don't let my cats outside. We have a big artesian well, and the cover over it is VERY flimsy. The ferals walk on it all the time and haven't fallen in...yet. But the well isn't full, and the walls are so steep, they could NOT get out if they did fall in.
Plus with raccoons (the mean kind...they have been attacking my feral colony, though I've heard 'coons can/will share feeding stations friendly-like), foxes, neighbor dogs, a busy street... I just don't let my kitties out without a harness & leash. And then hope they don't squirm out of the harness! LOL