What the legend says. Kallanai is traditionally credited to the Chola king Karikalan (Karikala Chola), usually dated to about the 2nd century CE, and it is often called one of the oldest water-regulating works in the world still in use. The version that travels online adds that it was built without cement and that scientists still cannot explain how it stands on a sandy riverbed. This lens draws it the way that story imagines it: one thin, seamless wall standing on the sand, holding back the Kaveri.

How it actually works. Kallanai is a low diversion weir, not a tall dam: about 329 m long and about 20 m wide, of unhewn stone and clay laid across the Kaveri's sandy bed near Tiruchirappalli. Its job is to hold the river back and divide it, so the delta's channels get water. The usual explanation of how it stands is plain engineering: big stones were laid on the bed, the current scoured the sand from under them, they sank and bedded in deeper, and more stones went on top, until the mass locked together by weight and friction. No cement is needed for that. What stands today is the ancient core under major later work: in the 19th century British engineers, notably Sir Arthur Cotton, studied it closely (Cotton is often quoted as saying its builders taught them how to found a work on loose sand), built the Upper Anicut at Mukkombu in the 1830s, and remodelled the Grand Anicut with sluices and a raised crest, drawn lit here. The flow now divides among the Kaveri, the Vennaru and the Grand Anicut Canal (Puthu Aru), with the surplus going to the Kollidam (Coleroon).