The Colosseum is a huge collapsed sinkhole, roughly 150 feet deep, created by the same karst processes that shaped Ha Ha Tonka's caves and natural bridge. The feature is reached from a signed state-park trail rather than by entering the collapse itself.
Filed under roadside oddities: giant objects, odd monuments and things built for no clear reason.
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Source: mostateparks.com/page/ha-ha-tonka-state-park/natural-history-ha-ha-tonka