The idea is not so off the wall but would suggest a spin on the method.
Not in a well, but buried a foot or so down in the back yard, being careful to maintain some math reference with other temporal markers so you can find it again when needed.
Moisture the culprit and you don't want to dig up your backup to find it dead rusted. The approach does makes it a bit more difficult to do incremental backups, but at least you got the last image secured.
If you got a Seal-a-meal type thing, do at least 3 enclosures on the drive with heavy mil plastic.
If it is a smaller passport unit, put it after enclosures inside a big glass bottle with a good sealing ring. Adam's peanut butter in the big jar is a choice.
Tighten the bottle and do an extra sealed bag enclosure for the bottle. Wrap the bottle in bubble wrap and bury it. The bubble wrap there to buffer earth shifts that could stress and break the glass.
Will be there longer than you will as far as needed. Outside the radar of some poor jerk ripping you off. Poor jerk because the thief has to face the bad karma with interest of the harm caused. There is no escape from the Law.
John