As I have been crossing this country I have attempted to find a cache in each county we drive through. That has not always been easy because finding a parking spot for a large RV within reasonable distance of a qualifying cache can be problematic. According to most sources there are 3142 Counties in the USA and this includes DC. Your challenge is to find a cache in at least 750 of the counties in the U.S.A., which would be about 25% of the counties.
A long TOTT or a hairy Tree Climb is required. If you can't replace where found don't try as that height is necessary to support the terrain rating.
Project-GC Challenge checker
A bit of history: 10 year ago I published a similiar challenge pre moratorium for 10% and other than myself almost no one qualified as few cachers were paying attention to counties. That cache now has ~35 finders as cachers are now paying attention county finds. Times change, in 2012 I could ask for a percentage number, today I'm requied to state a numer.