Ride through cave on Propane in Missouri. They "Show You" it works!
 

Fantastic Caverns, a roadside attraction. You ride through the cave in
a Propane-Burning, safe clean Jeep!

http://www.roadsideamerica.com/sight...ttrId=%3D13068

"Missouri is a state with more than its share of caves -- over 5,600 --
so Fantastic Caverns needed something a little different. In 1961, the
ride-through tours began -- post-WWII jeeps with gas powered engines.
The tours were popular, but cave walls don't fare well against car
exhaust. "People didn't care much back then," said our guide.
Eventually "America's Ride-Thru Cave" upgraded the jeeps to run on
cleaner propane."