How can someone extract and sequence DNA from something that's been buried for centuries?

Well it's actually extremely difficult. Cross contamination is a big problem, and so first of all you have to make sure that your sample is clean. If the person excavating it touched it, you don't want to end up extracting their DNA by accident. We bleach it to get rid of all the DNA on the outside and then have to try and find some cells inside the sample that are still whole. We then break those open and extract the DNA.