Skeptics argue that these apparent effects could result from anomalies
in local mass density, such as a hidden lode of metal ore. The
Greenland group, led by Mark E. Ander of the Los Alamos National
Laboratory and Mark A. Zumberge of the University of California at San
Diego, therefore chose a highly homogeneous site: a borehole
surrounded by a two-kilometer-thick expanse of ice. The team took
elaborate precautions: the bedrock was mapped by 42,000 high-frequency
radar scans, and careful surveys determined the height of the ice
surface to within a centimeter. A gravimeter took more than 100
readings at half a dozen locations, at depths of between 200 and 1,600
meters.



