The Last Spoiled Frontier

Now that we know there isn't an ancient city hiding on the moon, where am I supposed to have my bold adventurers go? Wait a minute, did I just write "on the moon?" I just checked, and I did. Sublunarian expeditions await! Until we do that too, at which point I dunno. Maybe something in Montana. Nobody checks anything that's claimed to happen there.

Addendum

Though if underground horrors keep popping out, the digging of holes might be highly regulated. Maybe people in that situation invent air golf.