How did the Curiosity rover weigh a mountain on Mars?

The Curiosity rover on Mars has some pretty amazing experiments aboard, but curiously enough never had a gravimeter, a device used for measuring the gravitational pull of buried rocks. So how then did scientists manage to weigh a mountain on Mars using years-old data taken by Curiosity? Charlie brings in a paper that reads like a data analysis version of a MacGyver episode. Read the paper and news stories here.