What in tarnation is a "tarn"? I never heard the word until I encountered it in "engines"
Episode 1010:

"Who remembers that Henry David Thoreau wrote
"Civil Engineer" after his name when he signed
documents? He fathered the American pencil
industry by inventing the clay binders that made
inferior American graphite into a decent pencil lead.
Then he manufactured and sold pencils. He also left
detailed drawings for his famous cabin by Walden
pond. Would-be ascetics have, ever since, made
perfect copies of his retreat in Colorado, Arizona --
wherever there's a mountain tarn or stream to
contemplate. "

I found it in a site about mountain climbing:
"The optimist in me was thinking "tarn... tarn... tarn," a term I guess I should explain.
A tarn is a small alpine lake, often fed primarily by snowmelt (the exact definition
varies). I knew already that one of them (using the loose definition) exists in southern California
It is unimaginatively named "The Tarn", and sits (in season) on the southern shoulder
of Mt. San Gorgonio. I hope to visit it someday."

tarn 7t9rn8
5ME terne < or akin to ON tj;rn, tarn, lit., hole filled with water6 a small lake, esp. one that fills a cirque

Geol. a steep, hollow excavation high on a mountainside, made by glacial erosion; natural amphitheater