To Ken3005...
Re: “AusMark - 17 and a half caves. Excuse my ignorance, but I don’t get it. Sorry”:

The Check-pants wearing tourist asked "How many undiscovered caves are there?"
AusMark’s ex answers: "17 and a half"
Ignoramis replies "How can you have half a cave?"

Therefore the joke is that they have so far only noted 17 and half undiscovered caves – it will be 18 undiscovered caves when they get round to it.

(Mind you, it could be that for the tourist to ask how many “undiscovered caves” there are is a bit dim. If they are “undiscovered” how could anyone give a numerically exact answer?)
(emadex thinks the world will end on, Tue 11 Jan 2005, 5:12, Reply)