Maithon is a large earth-and-masonry dam on the Barakar river, about an hour northwest of Asansol on the border with Jharkhand. The lake behind it stretches over 65 square kilometres at full capacity, with wooded islands and the Maithon Hills rising on the far shore.
The dam, the reservoir, and a small ecological park are open to visitors, and there is boating on the lake. It is an easy day trip from Asansol — most local taxis know the route — and a useful glimpse of mid-twentieth-century Indian infrastructure if you have any interest in the subject.