A visit to the Nilgiri Hills is mostly about enjoying the weather and exploring its parks and lakes. Rarely do we look deeper at the importance that nature has given these hills and continues to provide. Apart from the lush tea estates, gardens, misty forests, and tranquil waters of the lakes, a quiet, hidden world lies beneath. Deep within the mountain rock, an intricate network of carved tunnels, massive underground chambers, and heavy machinery operates entirely out of sight.
High in these scenic valleys surrounding Ooty, lies a remarkable engineering marvel: the Kundah Hydro-Electric Scheme. Hidden from our view, this sprawling network of reservoirs and subterranean powerhouses has silently balanced it's waters and generated electricity for Tamil Nadu for decades.
The story goes back to the early 1960s, when the Kundah hydroelectric projects were developed with assistance from Canada under the Colombo Plan. The first powerhouses in the wider Kundah Valley became operational in 1960.
A few years later, construction moved into the high-altitude forests. The Porthimund and Parsons Valley dams were completed by the mid-1960s, adding more reservoirs to the system.
For decades, these reservoirs ran a traditional, one-way gravity system, channeling mountain runoff waters down to the 30 MW Kundah Power House 6. Once those waters spun the conventional turbine and drained into the lower lakes, it was gone. Six decades later, these old reservoirs are taking on a new role in the age of renewable energy.
While the original Power House 6 continues its traditional role, TANGEDCO and contractor MEIL have built a brand new, parallel system right next to it. The 500 MW Underground Pumped Storage Project (PSP) creates a closed-loop recycling circuit using the staggered reservoirs and works like a giant rechargeable battery
Parsons Valley Reservoir at about 2,200 metres above sea level, acts as a feeder checkpoint, diverting the streams through an underground tunnel. This keeps the network balanced apart from supplying water to the Nilgiri district.
Porthimund Reservoir system, at about 2,220 metres above sea level, acts as the upper reservoir, holding millions of cubic metres of water.
Below, at around 1,985 metres, the Avalanche-Emerald reservoir system, form the lower reservoir.
The difference in elevation between the upper and lower reservoirs gives the system its stored energy.
When electricity demand rises, water is released from Porthimund and sent rushing downhill through large underground pipes. Its force drives reversible turbines deep inside an underground powerhouse, generating electricity before the water settles in the lower reservoir.
It is much like switching on a giant battery—except the stored energy is water held high in the mountains.
When demand falls, particularly during off-peak hours, the process reverses.
Surplus electricity from the grid is used to run the turbines backwards. They become powerful pumps, pushing the water uphill from Avalanche-Emerald lower reservoir back to Porthimund. The water is then ready to generate electricity again.
The same water can therefore move up and down repeatedly, allowing the system to store energy when it is available and release it when it is needed.
The most remarkable part of this project is that it preserves the local biodiversity completely intact. The landscape above remains an untouched haven of forests, lakes, and wildlife. Yet deep beneath it, water quietly flows through a hidden network of tunnels, storing energy and sustaining the power grid.
Sometimes, the most impressive engineering is not what we see.
It is what remains hidden, working quietly beneath our feet.
