You can find your location’s lat and long here. A little box at the bottom will tell you the lat and long. Enter that information here. Just add 180 degrees to your longitude and viola, your hole-digging destination.
I reasoned:
Sydney’s coordinates are: 33º 52’ South; 151º 12’ East
So “opposite” here must be: 33º 52’ North; 28º 48’ West
Okay, I think I’ve got it…
If I’m in Toronto, Canada, I’ll end up somewhere on Australia’s northeastern shore, yes?
I’d be in the south Atlantic, about a 800 miles directly east of Montevideo.
Lessee here. I am at 33.902 Lat, 35.484 Long; so my opposite is, if I know what I am doing, -33.902 Lat, -146.516 Long–directly between Australia and South America, absolutely in the middle of the Pacific Ocean. So I better bring my swim fins when I start digging.
I’d be in the Pacific Ocean - 1000km South East of Dunedin, New Zealand.
There is an island 180km to the North-NorthWest of me, but it looks uninhabited - at least, MapQuest doesn’t know what it’s called.
Grim
Okay. I’m at 43.636N , 79.488W
Flipping that over, we get 43.636S, 100.512E, an undistinguished stretch of water southwest of Australia.
Now, consider this. The town of Perth, Ontario, on Highway 7 west of Ottawa, is at 44.899N, 76.248W. Its opposite point is at 44.899S, 103.732E. Again, this is in the Indian Ocean SW of Australia… but the nearest large city to that point is Perth, Australia. 
I’d end up in the southwestern Indian Ocean, about a third of the way from South Africa to western Australia.
This site is the easiest way to do it. I end up in the Atlantic.
I knew that I should have taken that left turn at Albuquerque!
In the Pacific ocean, 500 miles or so south of Chatham Island (the closest land), itself 500 miles or so east of New-Zealand.
It won’t help. I did my location and another 250 miles north won’t have much of an effect. You wind up in the Indian ocean, about halfway between Africa and Australia.
Well, there we are. If you’d posted that site at the beginning, none of the rest of this thread would have happened!
I end up in the ocean southwest of Australia’s western coast.
Eeeek! Shaaarks!!!
Ok. So since I’m retarded how do I figure that out using coordinates?
My current location is 41 22’ 12" -83 38’ 49.2"
Okay, okay, so you’d be crushed into a very dense fleshball.
I forgot that on the SDMB, even hyperbole needs to be factually accurate 
Well, …no. Assuming conditions that could allow you to get down there in the first place, you would simply be weightless.
Moving the map around until you find the exit hole is “easy”?
Only if “conditions that could allow you to get down there” includes ways to survive extreme pressure…
That would put me in The Sea of…Tranquility. Interesting.