That is a 5ft section of a surveyor's ranging pole, that's not the deepest point. But five feet is about average overall.
It's a big hole ... estimate 15' x 15' and 5' deep - although it's not square in plan.
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A series of 18 images showing erosion and penetrations of the concrete cap, which has allowed scouring of the underlying fill leading to some significant voids and un-supported areas.