Mystery Cave, Forestville State Park

MN State Forestville State Park Site
Mystery Cave has over 13 miles of natural underground passages and is the longest network of caves in Minnesota. There are two entrances to the caves with guided tours for each. The "easy" tour lasts about one hour with 1/2 mile of walking on concrete walkways and modern lighting. The second tour is available only on summer weekends lasts about 2 hours with 3/4 miles of walking on gravel paths using hand held electric lanterns.

Mystery Cave was discovered in the mid 1930's when Joe Pettey recognized a portion of the hillside above the Root River had bare patches in the snow cover. Knowing that the caves maintained a constant temperature of 48o F all year long, he resoned that there must be air escaping from a cave entrance.

aerial view of Mystery Cave
Mystery Cave starts as a sinkhole is the Root River. In dry seasons the entire flow of the river can disappear just downstream from the cave entrance. The "hole" is not visible at the surface because it is "plugged" with larger rocks and gravel, causing it to look like the bottom of many other rivers. The space between the rocks is large enough to allow a good flow of water into the cave system.

The flow of the water, which is warm and contains carbon dioxide, (carbonic acid, just like in carbonated beverages) slowly dissolves part of the limestone rock and enlarges the passages over time. What starts as joints (cracks) in the limestone gradually becomes larger as the rock is dissolved, forming the caves.
cave diagram
cave diagram cave diagram
dye trace proceedure
Geologists know where the water goes by doing dye tracing. In the photo at left a geologist is pouring a dye into a sinkhole. Before the dye is put into the sinkhole, bags of activated charcoal are suspended in nearby springs.  After the dye is put into the sinkhole, the charcoal bags are tested at regular intervals for the presence of the dye. By doing this the geologists know not only where the water eventually  reappears above the ground but also how long it takes. From the dye trace reports in these other photos geologists have concluded that the water that enters Mystery Cave at this part of the Root River emerges at Seven Springs 8 hours later, bypassing a large loop of the Root River.

The Root River at Mystery Cave is a warm water stream, not good for trout fishing. The trout fishing starts at Seven Springs after water from Mystery Cave is cooled by the earth to provide the cool water that trout need to survive.
dye trace
dye trace dye trace
Notice that the caves aren't always easy to navigate. Here a photo of a cave explorer checking out "boxwork limestone" can just squeeze into the larger cavern. This part of the cave is not open to public tours but with the right training and work with at least two partners (so if someone gets hurt, one can remain while the other gets help) people can get permission to explore these parts of the cave system.
cave photo
Cave Entrance
Here a photo of the original cave entrance shows the layering of the limestone that is a typical feature of karst cave formations.


Credit for the spelling of Joe Pettey goes to his nephew, Lyle H. Pettey who found my error. Thanks

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