I'm pretty embarrassed by this...
One of the utility poles I used for my climbing wall was massive - based on a piece of it that I cut off, I estimate that it was nearly 2000 lbs. When I finally got it lifted & in its hole, I measured it to be sure that it was all the way in. Based on my calculations, it was in 5 1/2 feet, which was perfect.
The chain that I used to lift the pole was still attached way up high, so I played around on the chain (climbed it, took some swings on it, etc.). A few days later, I used another chain to attach it to my truck so I could straighten the pole up. When I pulled the pole with the truck, it dropped into the hole approximately 2-3 feet more. In other words, the pole wasn't nearly as deep in the hole as I had thought. The realization of that has given me a shiver or two.