Don't know what it is or how they do it.
Several years ago, I removed my ports for refinishing (sandblast, powdercoat) over the winter. Covered the boat with a tarp, which kept out all the rain and snow. Refinishing came out fine, but the tarp didn't keep out the cats! I still have no idea how they got onto and off the deck of the boat. Boat on the hard in a cradle. Fortunately, they just seemed to use it for sleeping, not a litter box! With StacyNeil's Zora, the boatyard cats had used the boat for a litterbox, ugh!