Here's some interesting data from the new Gravity sensing satellite.
Note the large blue (low gravity) blob at the north polar region. Seems to back up the thin ice area indicated in my earlier post. If you take the view that gravity is determined by the density of the earth beneath the surface object, then it makes perfect sense that a hole would show as a low gravity area.
The other point of interest on this diagram is that of the brownish lattice type lines. If anyone has any info on that then please let me know. Looks to me like they could indicate an electrical coil, Energy grid anyone?