From the Missouri State Parks website (https://mostateparks.com/content/geocaching-and-letterboxing)

Geocaching is a popular game using global positioning systems (GPS) to locate hidden containers, called caches. Letterboxing is similar, except clues, map coordinates or compass bearings are used to find hidden treasures. Both activities are permitted in Missouri state parks and are a great way to get kids and adults outdoors. There are many caches already placed in Missouri state parks. 

If you would like to place a cache in a Missouri state parks, please follow these steps and guidelines.

• Complete the geocaching/letterboxing placement permit through the following link: Official Geocaching/Letterboxing Placement Permit.
• Cache placement in recognized wild and natural areas needs to be kept to a minimum. Before placing a cache, please check with the facility manager for allowed areas. No digging or disruption of the ground is permitted when placing a cache. Caches are not to be placed in areas that could potentially cause danger to visitors trying to locate the cache (e.g., in trees, underwater, in potential flood areas, or in areas that require climbing on cliffs or bluffs).
• The outside of the cache container must clearly display the words "official geocache" or "official letterbox."
• An explanation of the geocaching/letterboxing activity must be included in the container.
• The cache must contain family-friendly content.
• A cache owner is limited to two permits in one faciltiy at one time.
• Permits are valid for 12 months from the approved date.
• Official geocaches must be submitted to geocaching.com for review and approval.
• Multi-caches are limited to five total sites, yet treated as one cache and require only one permit.
• If a non-permitted cache is discovered, the facility is to remove the cache, retain it at the facility office for 30 days, and, if possible, attempt to notify the owner of their actions. If contact is not made, the cache is to be considered abandoned property.