The cultural center Doornroosje in Nijmege, Netherlands, has been the site of lots of graffiti and street art since it was built. Paul DeGraaf calls it a "graffiti hall of fame." He noticed a spot where the paint had fallen off, which revealed how very thick the layers of paint had become over thirty years' time. DeGraaf removed some more paint and studied it, to determine how old each layer is.
See a gallery of images from DeGraaf's project at imgur. -via Metafilter