Tree spiking involves hammering a metal rod, nail or other material into a tree trunk, either inserting it at the base of the trunk where a logger might be expected to cut into the tree, or higher up where it would affect the sawmill later processing the wood. It is used to prevent logging by risking damage to saws, in the forest or at the mill, if the tree is cut, as well as possible injury or death to the worker. The spike can also lower the commercial value of the wood by causing discoloration, reducing the economic viability of logging in the long term, without threatening the life of the tree. It is illegal in the United States, and has been described as a form of eco-terrorism.