2 Answers

Since the Earth is a spheroid, its center (the core) is thousands of kilometres beneath its crust. On the surface, the point 0°, 0°, located in the Atlantic Ocean approximately 614 km (382 mi) south of Accra, Ghana, in the Gulf of Guinea, at the intersection of the Equator and Prime Meridian, at the coordinates of zero degrees by zero, is the "center" of the standard geographic model, as viewed on a map—but this selection of longitude meridian is culturally and historically dependent. The center of population, the place to which there is the shortest average route for everyone in the world, could be considered a centre of the world, and is located in the north of the Indian subcontinent, although the precise location has never been calculated and is constantly shifting.

From wikipedia.

Best answer I have found with explanation is here:

http://www.livescience.com/1608-scientists-closer-center-earth.html