Sasthamcotta: the lake that a demon left behind

Kerala is a land defined by water — backwaters, rivers, lagoons open to the sea. But its single largest body of fresh water is something quieter and stranger: an inland lake ringed by green hills, watched over by an ancient temple and a troop of sacred monkeys, and kept miraculously clean by a creature almost too small to see. This is Sasthamcotta Lake.

The largest freshwater lake in Kerala

Spread across some 375 hectares in the Kollam countryside, Sasthamcotta is the biggest freshwater lake in the state — and not a decorative one. It is the drinking-water source for roughly half a million people in the Kollam district, and it supports fishing besides. In 2002 it was recognised as a wetland of international importance under the Ramsar Convention, one of only a handful of such sites in Kerala. It is, in short, a working wonder as much as a scenic one.

The larva that keeps it clean

And here is its quiet marvel. The water of Sasthamcotta is famously pure and sweet — and the reason, remarkably, is a tiny larva. The lake teems with a larva known as cavaborus, which feeds on the bacteria in the water, cleaning it as it goes. Where most large lakes need heavy treatment to be drinkable, Sasthamcotta is kept fit to drink by an ecosystem doing the work itself: a whole city’s water purified by an insect no bigger than a comma.

The god and the demon

The lake takes its name from the Sastha — Dharmasastha, Lord Ayyappa — whose ancient temple stands on its bank; Sasthamcotta means, roughly, “the fort of the Sastha.” Legend holds that where the lake now lies there was once a thick and terrible forest, the lair of a dreadful demon named Bana. It was Ayyappa, the story goes, who came and slew the demon, and the great hollow of the lake was left behind by that cosmic struggle. The temple has stood guard over the water ever since.

The sacred monkeys

No visitor forgets the monkeys. Large troops of them live in the temple grove along the bank, and by local belief they are no ordinary animals: they are held to be the close aides and companions of the presiding deity, the Dharmasastha himself, and are treated as sacred. To watch them move through the trees above the still water, half-wild and half-worshipped, is to feel how completely nature and faith are braided together here.

Visiting Sasthamcotta

The lake lies in the countryside a short way from Kollam town, an easy and peaceful half-day trip, with boating on the water and the hilltop Sastha temple above. Treat the monkeys with respect and keep your food and belongings close. Details are on the Sasthamcotta place page; more of the district is on the Kollam hub.