Gravity anomalies. Coloured free-air gravity anom- aly map of the Earth, centred on the Atlantic Ocean. These anomalies are the differences between the theoretical value for the gravity at the sur- face & the measured value. The colours range from purple (low gravity), through blue, green (normal), yellow, red to white (high). The anomalies are only a tiny fraction of the gravitational field strength, but they can provide information on the Earth's internal structure. The gravity low in Hudson Bay, Canada (upper left), occurs partly because the area's rocks are recovering from being compressed during the last ice age. This photographic print is digitally printed on archival photographic paper resulting in vivid, pure color and exceptional detail that is suitable for museum or gallery display.