In the 60's it was a top secret Lockheed facility attempting to develop nuclear powered aircraft. Later, after they failed to make progress, they repurposed the facilities (which included a small nuclear reactor underground) to research the effects of ionized radiation on various objects, flora, and fauna.
They had a railway system set up throughout the forest, and would fill traincars with various things, then roll them by the nuclear reactor, which could be raised up out of the ground, to expose the surrounding area to radiation. Nowadays it is a state forest, completely open to the public, with horse and foot trails. To this day, there are several hot spots that are still radioactive and fenced off (poorly).