Listed by Unesco as a 'masterpiece of creative genius', the Shushtar Historical Hydraulic System has been diverting water for irrigation from the Karun River for over a millennium. The system comprises bridges, weirs, canals and tunnels, but the most impressive component is a series of ancient watermills powered by human-made waterfalls. The mills are situated in a tight ravine and the cascading waterfalls, sluices and millpond are floodlit at night. The entrance is down steps just before the Shari'ati St bridge.
You can see the waterfalls almost as well from the Shari'ati St bridge (300m southeast of 17 Shahrivar Sq) or, even better, by climbing the hill on the downstream right; follow signs to Marashi House. However, nothing beats getting up close to fully appreciate the force of the water and the skill of the ancient engineers.
The hill is a prime cafe perch, so keep your eyes peeled for a chay with a view.