[adventured]

Dramatically overfill it with concrete - enough that it's wildly impractical to ever remove it - and put a giant stainless steel monolith in the middle.

Nature will rapidly disappear and or anonymize your earth hole, or someone will simply fill it in (just one person digging with industrial equipment will make a surprisingly modest dent in the earth in a lifetime). You need to make it obnoxiously unreasonable to bother getting rid of it.