▁than ▁fifty ▁bill boards ▁along ▁Inter state ▁ 4 4 ▁and ▁is ▁considered ▁one ▁of ▁the ▁primary ▁attra ctions ▁along ▁former ▁U . S . ▁Highway ▁ 6 6 . ▁Mer ame c ▁Ca ver ns ▁is ▁the ▁most - vis ited ▁cave ▁in ▁Missouri ▁with ▁some ▁ 1 5 0 , 0 0 0 ▁visitors ▁ann ually . ▁Mer ame c ▁Ca ver ns ▁is ▁ranked ▁# 1 7 8 ▁on ▁Ca ver Bob . com ' s ▁USA ▁Long ▁C ave ▁list . ▁ ▁History ▁▁ ▁The ▁Mer ame c ▁Ca ver ns ▁have ▁existed ▁for ▁the ▁past ▁ 4 0 0 ▁million ▁years , ▁slowly ▁forming ▁through ▁depos its ▁of ▁lim estone . ▁In ▁centuries ▁past , ▁Native ▁Americans ▁used ▁the ▁ca vern ▁system ▁for ▁shelter . ▁The ▁first ▁cave ▁west ▁of ▁the ▁Mississippi ▁River ▁to ▁be ▁expl ored ▁by ▁Europe ans , ▁it ▁was ▁first ▁visited ▁in ▁ 1 7 2 2 ▁by ▁a ▁French ▁min er . ▁ ▁During ▁the ▁ 1 8 th ▁century , ▁the ▁cave ▁was ▁used ▁for ▁extract ing ▁salt p eter ▁for ▁the ▁manufact ure ▁of ▁gun pow der . ▁In ▁the ▁Civil ▁War ▁era , ▁the ▁Union ▁Army ▁used ▁the ▁c aves ▁as ▁a ▁salt p eter ▁plant , ▁but ▁the ▁plant ▁was ▁discovered ▁and ▁destroyed ▁by ▁Confeder ate ▁gu err illas , ▁likely ▁including ▁the ▁future ▁inf am ous ▁out law ▁J esse ▁James . ▁According ▁to ▁local ▁legend , ▁James ▁and ▁his ▁brother ▁and ▁partner ▁in ▁crime ▁Frank ▁used ▁the