I lifted this all from Paul Arlinghaus' answer to a question about tetanus hill. This was such a great history lesson I thought I'd save it for posterity 
The property was farmed and then quarried for gravel. In order to protect the quarrying operation a levy was built. The levy was often built by piling trash (farm fence, old cars, misc equipment, metal trash from the quarrying, etc... and then covering it with dirt. This was in the late 1800's or early 1900's.
In 1913, there was the largest flood on record in Indianapolis. The flood was caused in large part by all the flood plains along the White River that had been blocked off by levies. Many of the levies were broken after the flood to allow the flood plains to do what they are supposed to do (disperse flood waters).
So the land that is now Town Run Trail Park has been significantly altered from its natural state.
To fully illustrate the amount of man made changes to the land, look at the next three photos of Town Run South.
Town Run South 1941
Town Run South 1972: Notice the land removed at the oxbow. This was a gravel operation and the land around the edge was left to act as a levy.
Town Run South 2010: Only a faint outline of where there used to be land.
The point is that Town Run has a long history. And some of that history is evident in the metal objects found in the the ground. There are scattered cables, old farm fence, glass dumps, buried cars, etc.....
So in cases of places like Tetanus Hill, the car buried in the ground is part of Town Run's history.
Due to the amount of human manipulation and the amount of man made objects buried in the ground, there is no way to remove all such objects. In the case of the car buried in Tetanus Hill, it would take a bull dozer to remove the car, and then there would be no hill left to build a trail.
It also is very dangerous to start removing metal objects that become exposed. A small piece of metal or wire, could be attached to a bail of farm fence, a car, or large tangled mess of metal. So in general if metal objects don't have sharp edges or pose the same risk as natural objects like rocks, logs, sticker bushes, etc.... then we leave them alone.
We do look for sharp metal objects along the edge of the trail and try to remove them as we find them. Keep in mind that these objects tend to work their way to the surface due to natural events like freeze thaw, floods, droughts, etc... So we will likely never see and end to new objects coming to the surface.
So in the case of Tetanus Hill, we will not be removing all of the metal parts in the trail tread. It is a part of the history of Town Run and gives the trail character. It also would create a more hazardous condition if we started digging out parts of the car, as we would just exposed more of the car. I will look at the area and remove any sharp objects that pose risks above the normal risks involved in mountain biking
Thanks,
Paul
