The fossilization process, which takes place along millions of years, consists basicaly in the replacement of the remains of an organism components by minerals.
Sometimes, fossils are not remains but evidence of the activity of live beings, such as burrows, footprints, track ways or perforations.
Some fossil findings have taken place by chance. But, as most fossils are generally buried, it may be tough to decide the exact point for an excavation.
source: World Images