Weight Measured underground

It is often stated that weight decreases linearly with depth inside the Earth, reaching zero at the centre. That result is exact only for a body of uniform density. This paper derives the near-surface depth gradient of gravitational acceleration for a spherically symmetric body, dg/dd = 2g/R − 4πGρ_local, and shows that for Earth's crustal density, which is below two-thirds of the mean density, g increases with depth. It then reviews Airy's 1854 Harton Colliery experiment, mine and borehole gravity surveys from the 1970s and 1980s, and the Greenland ice-sheet borehole experiment, in which observed gravity rises by 338.5 mGal between 213 m and 1673 m depth. None of the sources reviewed reports a measured decrease of weight with depth. Behaviour below a few kilometres, including the maximum of g near the core–mantle boundary, is inferred from seismic density models and has not been tested by direct weight measurement. This is a targeted review, not an exhaustive survey of the literature.