▁was ▁a ▁major ▁western ▁out post ▁of ▁Mississ ipp ian ▁culture , ▁which ▁domin ated ▁the ▁Mississippi ▁Valley ▁and ▁its ▁trib ut aries ▁for ▁centuries . ▁S pi ro ▁M ounds ▁is ▁under ▁the ▁protection ▁of ▁the ▁Oklahoma ▁Historical ▁Society ▁and ▁is ▁listed ▁on ▁the ▁National ▁Register ▁of ▁Historic ▁Places . ▁ ▁In ▁the ▁ 1 9 3 0 s ▁during ▁the ▁Great ▁De pression , ▁tre asure ▁hun ters ▁bought ▁the ▁rights ▁to ▁tunnel ▁into ▁Craig ▁M ound — the ▁second - larg est ▁m ound ▁on ▁the ▁site — to ▁mine ▁it ▁for ▁artifact s . ▁They ▁exposed ▁a ▁h ollow ▁bur ial ▁chamber ▁inside ▁the ▁m ound , ▁a ▁unique ▁feature ▁containing ▁some ▁of ▁the ▁most ▁extraordinary ▁pre - Col umb ian ▁artifact s ▁ever ▁found ▁in ▁the ▁United ▁States . ▁These ▁included ▁works ▁of ▁frag ile , ▁per ish able ▁materials : ▁text iles ▁and ▁fe athers ▁that ▁had ▁been ▁uniqu ely ▁preserved ▁in ▁the ▁conditions ▁of ▁the ▁closed ▁chamber . ▁The ▁tre asure ▁hun ters ▁sold ▁the ▁artifact s ▁they ▁recovered ▁to ▁art ▁collect ors , ▁some ▁as ▁far ▁away ▁as ▁Europe . ▁Some ▁of ▁these ▁artifact s ▁were ▁later ▁returned ▁to ▁regional ▁museum s ▁and ▁the ▁C add o ▁Nation , ▁but ▁other ▁artifact s ▁have ▁never ▁been ▁account ed ▁for . ▁This ▁site ▁has ▁been ▁significant ▁for ▁North ▁American ▁arch ae ology ▁since ▁the ▁ 1 9 3 0 s , ▁especially ▁due ▁to ▁its ▁many ▁preserved ▁text iles ▁and ▁wealth ▁of ▁shell ▁car ving .