your gravity vector changes(very slightly) depending on the density of the material around you.
there were some spiral tunnels being built 100+ years ago going through the mountains that didn't take this into account. they blasted the tunnel out from both ends and they didn't line up in the middle where they had planned to line up.
ooo interesting, where were these tunnels built?
spiral tunnels where the Canadian Pacific railway crosses the Rockies. famous story for surveyors. all our instruments depend on gravity being down to the center of the earth. but what happens when you have a mound of ironish minerals on one side of your project? lots of investors money being lost is what. Back in the days when it was honest mistakes losing you money.
