Comment by londons_explore

By modelling the previous location of groundwater (under the ground), and the new location of groundwater (on the surface of the ocean) one can calculate the change in the earths rotational axis that is expected. The measured change matches predictions only if this effect is considered.

The location changes the axis because most land is on one side of the earth (centered around Europe), and most ocean is on the other side (centered around the south Pacific). Move mass from one place to another, and the axis changes.