Crane Creek flows right to left in front of me, spring-fed and uncommonly clear and cool, slicing through trees that line both banks. Shallow and thin, it runs 25 miles through thick forests in southwest Missouri, at some points no farther across than a fishing pole is long. You could walk across it and never get your knees wet.

I arrive here on a Tuesday with my friend Aaron, two fishing poles, and the hope that in two days here we will see (never mind catch) a mysterious fish that has fascinated anglers for generations — one of the beautiful rainbow trout that hide under the ledges and logs in the water.

They are fireworks with fins: Their green freckles, pink stripe, and silver body pop far more vividly than rainbow trout found in other Missouri streams. These rainbows are a coveted trophy in the fly-fishing community, not just because of their beauty, but because of their wildness, how difficult it is to get one on the line, and how hard they fight once you do.

Also: their backstory. You won’t catch another fish with an ancestral heritage like this one. According to legend, the rainbow trout in Crane Creek represent a genetically pure strain of McCloud River redband trout, originally from California and now thriving in this hard-to-reach spot and only a few other places on Earth. To catch one is to come face to face with a mystery: Is the legend true? Are the trout pure? What am I supposed to call these fish?

This is a question not just for the people who fish in Crane Creek, but for every scientist working to name every living thing on Earth — and discovering, with technology’s help, that there are more and more things to name.

Read more on Experience Magazine.

Illustration by Angela Pyne.