http://en.wikipedia.org/wiki/Antipodes

The antipodes of any place on the Earth is the place that is diametrically opposite it, so a line drawn from the one to the other passes through the centre of the Earth and forms a true diameter. For example, the antipodes of New Zealand’s lower North Island lies in Spain. Most of the Earth’s land surfaces have ocean at their antipodes, this being a consequence of most land being in the land hemisphere.