What is the most unique or hardest geocache you have found?

Some of you might not know about geocaching, what it basically is it’s a treasure hunt but instead of keeping the treasure you have to trade something of equal or greater value and then sign the log.
The 2 most unique ones I have found are an ATM and you have to crack open the safe to get inside but they give you 3 of the numbers and you have to find the fourth one
the second most unique one I have found is one that’s a rock but you have to pull it out of the ground and you’ll see it’s attached to a tube and you open up the tube to find the log book and stuff