The redwood groves in Northern California are one of America's greatest natural resources. Redwoods can live more than 2,000 years and grow taller than 350 feet. When tourism became popular in the late 1800s, large tunnels were cut through the center of several of these giant trees to allow wagons to pass through (and to attract tourists to the location). Even today, visitors wait in line to be photographed driving their cars through a tunnel. When the groves were converted into national parks, the tunneling was discontinued, but several tunneled trees are still alive.

When redwood tunnels were first constructed, not much consideration was given to the long-term effects of a tunnel on a tree's health. Nevertheless, many trees have survived more than 100 years after the tunnel was cut through their trunks. This is possible because: