Silent Valley National Park in Kerala's Nilgiri Hills hosts one of India's last undisturbed tropical rainforests. In the 1970s–80s, a proposed hydroelectric dam on the Kunthipuzha River threatened its ecosystem. A mass environmental movement, backed by the Kerala Shastra Sahitya Parishad and poet Sugathakumari, pressured Prime Minister Indira Gandhi to shelve the project. Silent Valley was declared a National Park in 1980 under the Wildlife Protection Act, 1972, and later became part of the Nilgiri Biosphere Reserve.

Silent Valley represents a landmark convergence of ecological science and democratic environmental activism in India. The park shelters one of the world's rarest tracts of tropical wet evergreen forest — what ecologists call 'sholahs' — in the Western Ghats. It is home to the lion-tailed macaque (Macaca silenus), an IUCN Endangered species, along with 34 mammal species, over 400 bird species, and extraordinary insect diversity.

The Western Ghats, of which Silent Valley is a jewel, is one of the 36 globally recognised biodiversity hotspots (as per Conservation International's framework). A biodiversity hotspot requires ≥1,500 endemic vascular plant species and has lost ≥70% of its original habitat.

The Silent Valley movement is historically significant because it predated India's Environment Protection Act, 1986 — the dam controversy was a catalyst for strengthening environmental legislation. The Kunthipuzha River, a tributary of the Bharathapuzha, would have been dammed for a 120 MW hydroelectric project by Kerala State Electricity Board.

Silent Valley was declared a National Park in 1980, and in 1986 it became part of the Nilgiri Biosphere Reserve — India's first biosphere reserve, designated under UNESCO's Man and Biosphere (MAB) programme in 1986. The Nilgiri Biosphere Reserve was also the first Indian site inscribed on the World Network of Biosphere Reserves.

For exam purposes: connect Silent Valley to (1) biodiversity hotspot classification, (2) UNESCO MAB programme, (3) lion-tailed macaque conservation, (4) the legislative history — Wildlife Protection Act 1972 and Environment Protection Act 1986.