Like so many caves around it, the The 17 Mile Cave is a lava tube and a relic from Idahoe's volcanic past (although the area is still very active). They are formed when eruptions cause a lava crust to form, cool down and allow lava to flow through again. Its name comes from the proximity to the town of Idaho Falls. The tube is around a quarter mile long.
The entrance is located directly on the West Arco Highway and is accessible to everyone. This is probably one of the reasons why this fantastic natural monument has fallen victim to modern cave "paintings". If you're ever in the area, make sure you bring several light sources and a jacket. It's really cool in there.