It was a muddy area in the infield noted for drugged and drunken debauchery. People would drive old cars there and set them on fire, creating a cheerful blaze which set the autumn night alight. As the years ensued, participants took to stealing cars and setting fire to them.
The climax came one year, I believe it was '74, in which a charter bus from Brazil was stolen. It was an impressive sight indeed. When the crowd was done with it it was on its roof and compressed to half its former height.
Another interesting Bog practice was "The Other Side Sucks." Imagine a crowd of hundreds surrounding a pile of burning cars (and a bus in one case). The crowd, being shaped like a doughnut, had an infinite number of "other sides." So soon stones and beer bottles would describe graceful arcs in the firelit night sky, only to crash into the people on "the other side." Unsurprisingly, injuries were frequent but I a unaware of any fatalities. A lone cop car entered the Bog but was met with a volley of beer bottles. It beat a hasty retreat, rear window smashed out.
This much fun was destructive and couldn't last. Towards the end of the Seventies the Bog was paved over and the police presence was increased. This put an end to the festivities at the Bog and a level of danger and participatory hooliganism unimaginable to modern race fans.