Petrification is the fossilizing process in which organic material (such as wood) changes into stone. Petrification is caused by mineral-rich water seeping into the empty spaces of dead, buried trees or animals, which eventually become stone. Although petrified remains can be up to 300 million years old, the stone often reproduces the original living material so clearly that the cell structure is identifiable. The Petrified Forest National Park in Arizona, has many examples of petrified wood.