The Earth does not have uniform density, therefore the force of gravity from
the Earth's mass is not quite the same in all directions. (you may have read this before somewhere)
There is an area over the Andes Mountains where gravity is somewhat weaker.
This is referred to as "gravity hill". Satellites over this region require constant attention, as they will drift out of orbit easily.
-it's a fact!
thanks to Fruan, who suspects something may have been smuggled through his airport and inspired me to node science facts today.