According to reports, this tunnel goes through a hill in Pennsylvania and was initially built for trains. It was then turned into a normal highway but has since been closed and is now owned by Floyd Ganassi Jr, also known as Chip, a former race car driver and current racing team owner. He uses the tunnel to test various race cars for acceleration and then lets them roll to test aerodynamic properties.
The tunnel is said to be around 1.6km long and features a turntable at the end so Chip can spin around and roar back again. This is surely one of the coolest setups going around. For more information on the tunnel, take a look at this link.