Abandoned Train Stations (US)

While there's no great lost continents left to discover, human expansion provides. Multiplying at an exponential rate, building over and around what already exists, fossilizing the past for future generations to discover.

I've always kept sites I've found myself secret, and I never myself publish anything that I think might be undiscovered or otherwise sensitive (the location of the Golden Tunnel, for instance). I only repeat what's already been made quite public. The less forgotten these places become, the more likely it is that the proper authorities will come bearing padlocks, barbed wire, and security cameras.

That said, abandoned train stations & tunnels from NYC. The same rules apply when you're trekking through the mountains, "Take only photos, leave only footsteps." Don't be an jerk.