At one of our weekly maintenace clients we moved a small standard tree recently that wasn't doing particularly well and was too close to other plants. It came up very easily and when we moved it there was a hole underneath.
I shone my phone torch into the hole but couldn't see anything apart from darkness.
So, we put a spade into the hole but couldn't feel the bottom
So we pushed our rake handle down the hole..... but couldn't feel the bottom!
So, I put my phone in the hole and took a photo and this is what we saw...
An approx 4 foot wide and 11 foot deep well!