1: Notionally speaking, where you would wind up if you proceeded to dig a hole through the earth as to come out the other end. (This is mistaken, of course - starting from either coast of the US would put you in the middle of an ocean, as would starting about anywhere in China.) See china syndrome
for a possible explanation to the origin.
2: Metaphor for where one can see if they have a clear view of a woman's genitalia - either due to the fact that she is wearing no clothing, or the clothing she is wearing is revealing what's underneath (due to transparency or just that it is ill fitting). Possibly originates by extension of sense 1.
When she lifted up her legs, we could see all the way to China.