I'm not sure the actual depth, I have dug graves for mostly guineapigs in my time, but I would say at least 3 ft ? I would get as deep as you can go to be honest, otherwise (sorry to say this) but other animals can smellthem, like dogs and foxes and could dig them up. I would also suggest, after the bunny is buried, that over the fresh soil you place a large paving slab, rock or some wire netting...anything that stops animals digging in the soil for a few months. Sorry you are having to do this, it is a horrible experience isn't it.