I pride myself in making good geocaches. Some are whimsical. Some are difficult. Some are both. And on occasion they actually get better with age.
I've done several using plate fungus as a base. Now, I'm not saying where this one is, but if you were to look very closely at it you might just see the bit of black plastic that holds it to the piece of dead wood below. But it's not as easy as it was on first deployment. Because the plate fungus has dropped spores that are now growing on the supporting structure!
I've given this one five stars for difficulty.