Don't drink from Mammoth Spring, says the sign. It's full of dissolved inorganic nitrogen compounds that leech into the water from the minerals and "other sources." Here's what I think those other sources are -- bat guano. The first time I went spelunking as an adult, one of the members of my party told me not to drink cave water no matter how clear it looks. He pointed at bats clinging to the ceiling and then to the pool of cavewater beneath.
Mammoth Spring flows into a pond where huge mattes of algae feed off of and thus deplete the high concentrations of those same compounds. From there, the water dumps into the Spring River at the rate of 9.5 million gallons an hour. That's one of the biggest springs in the world, and it affords this area with year-round trout fishing even in the heat of August when the Buffalo river is only half-floatable.
Mammoth Spring lies on highway 63 at the Missouri border.