History · Engineering · 8 min read

Moolamattom: The Power Station Hidden Inside a Mountain

Beneath the quiet green slopes of Nadukani, at the end of a tunnel cut through solid granite, six giant turbines spin in a cavern longer than a football field. The story of Kerala's greatest power station, and the mountain that keeps its secret.

By GK Global, Adimali, Munnar · Trails & Tales by GK

From the road that climbs out of Moolamattom, the hill of Nadukani looks like any other in the Western Ghats. Mist settles in its folds in the mornings, hornbills cross its ridges, and in the evening the light turns its cliffs to copper. Below, the town goes about its day: buses grinding up the hairpins towards Idukki, children walking home from school, the smell of tapioca and fish curry from roadside hotels.

Nothing on the surface tells you what lies inside. But walk into the mountain's heart, along a tunnel about 600 metres long, and the green world disappears. The air turns cool and still, the walls glisten with damp, and the only sound is a low, steady hum, growing louder with every step. Then the tunnel opens into a hall of light: a vast cavern carved out of living granite, where six great machines spin day and night. This is the Moolamattom Power Station, the beating heart of the Idukki Hydroelectric Project, and on many days the single largest source of electricity in Kerala.

An Indo-Canadian dream

The idea of harnessing the Periyar at Idukki is older than independent India. In 1922, an Oorali headman named Kolumban led two hunters to the narrow gorge between the hills of Kuravan and Kurathi, and ten years later one of them, the estate superintendent W. J. John, proposed a dam there to the Travancore government. (Read the full story in The Titans of the Ghats.)

The dream became real only in the 1960s. The project was sanctioned in 1963, and the Government of Canada came forward with long-term loans and grants under the Colombo Plan, the Commonwealth programme of aid to Asian countries. The Canadian consulting firm SNC (later SNC-Lavalin) advised the engineers of the Kerala State Electricity Board, while the Hindustan Construction Company took on much of the building.

Their task was enormous. Three dams, at Idukki, Cheruthoni and Kulamavu, would trap the Periyar and its tributaries in a reservoir of some 60 square kilometres. But the real prize was not the lake. It was the drop. Just west of the reservoir, the Western Ghats fall away steeply towards the plains of central Kerala. If the water could be led through the mountain to its foot, it would fall more than 650 metres, a head few power stations in India could match.

Hollowing out a mountain

The engineers decided to put the power station not at the foot of the hill, but inside it. An underground station is shielded from landslides, floods and monsoon storms, keeps the steep, high-pressure pipes short and safe, and, in a time of war and uncertainty, is almost impossible to attack.

So, from the late 1960s, crews began to blast their way into Nadukani. They drove an access tunnel into the hillside, and then, deep inside, they began to excavate the main hall, drilling, charging and firing the granite round after round, hauling out the broken rock in trucks, bolting and lining the walls as the void grew. Alongside it they carved smaller chambers, shafts and galleries for valves, cables and ventilation, and a long tailrace tunnel to carry the spent water away.

The result is a cathedral of stone: a hall about 141 m long, nearly 20 m wide and almost 35 m high, longer than a football field and tall enough to swallow a ten-storey building. When it was completed, Moolamattom was hailed as the largest underground power station in India, a title it held for many years and one that is still often repeated. Since then, larger underground stations have been built in the Himalaya, such as the 1,500 MW Nathpa Jhakri in Himachal Pradesh, but Moolamattom remains one of the largest in the country, and by far the largest power station in Kerala.

The first machine turned on 4 October 1975. On 12 February 1976, Indira Gandhi dedicated the first stage of the project to the nation, and the remaining machines were added in the years that followed, bringing the station to its full 780 MW. In February 2026, Kerala marked fifty years of the Idukki project.

The anatomy of power

To understand Moolamattom, follow a single drop of water.

Near Kulamavu, it is drawn into an intake and enters a power tunnel about seven metres wide, bored for kilometres through the granite. In the darkness it flows west, steadily, under the forest and the hills, until it reaches a surge shaft, a vertical well that absorbs the shock when the turbines speed up or slow down.

Then the ground falls away. The water plunges into two steel-lined pressure shafts, each nearly a kilometre long, that drop steeply through the mountain towards the cavern. By the time it reaches the bottom, it has fallen about 669 metres, more than twice the height of the Eiffel Tower, and it is under immense pressure.

At the turbines, the pressure is released through nozzles as a jet of water travelling, in theory, at over 100 metres a second, roughly 400 km/h. The jets strike the cup-shaped buckets of the Pelton wheels, the type of turbine best suited to very high heads, and spin them at 375 revolutions a minute. Each wheel turns a generator of 130 MW. Together, the six can produce up to about 18 million units of electricity a day, and in an average year the station supplies a large share of all the power generated in Kerala.

Spent and calm again, the water drops into a channel beneath the machines and leaves through a tailrace tunnel about 1.2 km long, emerging into daylight near Moolamattom to join the Thodupuzha river, a tributary of the Muvattupuzha. Thanks to this steady flow, the river now runs full all year. Downstream, it fills the Malankara reservoir, which drives a small power station and feeds irrigation canals across the plains of central Kerala.

The fortress of light

For most people, Moolamattom will remain a mystery. The power station is a restricted installation, guarded by the KSEB and the police; casual visitors are not allowed in, and entry is granted only with special permission, usually for official, technical or study visits. Photography is forbidden.

Those who have been inside describe the journey in much the same way. The vehicle, or your own feet, carry you into the hill along the access tunnel, out of the warm, green daylight into a cool, echoing passage lit by a line of lamps. The hum grows. Then the tunnel opens into the great hall: clean, brightly lit and astonishingly large, its rock ceiling high overhead, a travelling crane spanning its width. Along the floor stand the six generators in a row, painted and polished, each with its control panels and gauges, while engineers in helmets move between them. You feel the power more than you hear it: a deep vibration in the floor and the air, the sense of a vast weight of water falling somewhere just beyond the walls.

It is a place of quiet heroism too. In November 2025, while the station was shut down for maintenance and the water drained away, a team of KSEB dam-safety engineers walked about 1.5 km into the empty pressure tunnel to inspect it, in low oxygen, keeping in touch with the surface through ham radio.

There is a sobering side to the mountain's secrecy. The single access tunnel is the only way in and out of the powerhouse, and safety experts have long pointed out how difficult an evacuation would be in an emergency. Like everything at Moolamattom, the station's safety depends on careful engineering and constant vigilance.

Moolamattom and the hills that hide it

Moolamattom itself is a small, busy town at the foot of the Ghats, in the valley where the tailrace water emerges. It grew up with the project: its KSEB colony, offices and workshops date from the days of construction, when engineers, labourers and their families from across Kerala and beyond came to live here.

The best way to understand what lies inside the mountain is to see it from above. About 4 km beyond the Kulamavu dam on the Thodupuzha–Idukki road, at roughly 900 metres, stands the Nadukani pavilion, a circular, two-storeyed viewing tower with a domed roof, built by the KSEB in 1966, when the project was taking shape below. From its gallery and the boulders around it, the land falls away to the west in a vast sweep of green: the town of Moolamattom far below, the gleam of the Malankara reservoir, the river winding out towards the plains, and ridge after ridge of the Western Ghats fading into the haze. Somewhere beneath your feet, the water you cannot see is falling through the mountain to the turbines.

(Read more about Kerala's water power in the Pallivasal story, Ponmudi Dam and the Anchuruli tunnel, which brings extra water into the same reservoir.)

Visitor's guide

• The power station: closed to tourists. Entry requires special permission from the KSEB and is rarely granted to the general public. Do not attempt to approach the tunnel entrances or photograph the installations.
• Nadukani pavilion: run as a viewpoint by the KSEB's hydel tourism wing, generally open from morning to evening for a small fee; timings and fees change, so check locally. Clear days between October and February give the best views; mist can hide everything in the monsoon.
• Getting there: Moolamattom is about 20 km from Thodupuzha; Nadukani is about 32 km from Thodupuzha on the road to Idukki, via Moolamattom and Kulamavu. The road is steep, with many hairpin bends; drive slowly, especially in rain and mist.
• Nearby: the Idukki and Cheruthoni dams, open to visitors at set times; Kalvari Mount, for a view over the reservoir; the waterfalls of Thommankuthu near Thodupuzha; the hill of Ilaveezhapoonchira, a short drive from Moolamattom; and the Malankara reservoir. (See also the waterfalls of Idukki.)