South Georgia Islands and Antarctica in 1998 - one of our fellow passengers wrote about the trip in a book, which is nice to have (let's say Peter Matthiessen is eminently more erudite than I) in End of the Earth: Voyages To Antarctica.

It didn't even take me seconds to think about this, though I have traveled in nearly 100 countries on the six other continents and scuba dived in >30 of them.