I was almost single-handedly responsible for bringing a plague of flies!
How gross is this? If you look at the second photo, you can see some of the more enterprising wrigglies trying to escape over the side.
I keep a litter tray in the garden, which they've never used once. Been there through the wet, then sunny weather with compostable litter. Took the lid off, and found a foamy, crawly maggot gravy! 
Dumped a load of bleach in there, but DH's gone out for more. I'm currently keeping watch for any escapees, armed with a trowel. Then down the drain would you reckon?