Qanats are a fascinating engineering feat requiring a precise slope of 1:1000 or less, built over 2000 years ago. It is really amazing what these ancient civilisations built with the technology available.
ETA: Also one of the few scrabble words with a "q" but no "u" :-)
"Most qanats in Iran run less than 5 km (3.1 mi), while some have been measured at ≈70 km (43 mi) in length near Kerman" --https://en.m.wikipedia.org/wiki/Qanat
I'm from that region with lots of Qanats and I remember we used to fish in them. Thinking back, I now realise how awesome it was to do that. Always curious how the fish ended up there.
