More than 100 meters deep, Finland has begun excavating three giant caves to store 1.1 million cubic meters of water at up to 140°C and to retain summer heat for heating cities in winter.

Written by Noel Budeguer

Published on 24/03/2026 at 11:07

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With water at up to 140°C, three giant caves and a capacity of 90 GWh, the project started in Vantaa places Finland in the race for underground heat solutions and changes the strategic outlook on urban heating

Finland has begun to make room underground for a project that draws attention for its size and function. In Vantaa, three giant caves are being excavated to store heated water and form an underground heat reserve for the colder months.

In practice, the project creates a kind of thermal vault capable of storing energy in the form of heat when demand decreases and releasing this volume in winter. The expected effect is to strengthen the urban heating network and reduce pressure during peak consumption periods.

Three caves with 1.1 million m³ will store seasonal heat

The project plans to construct three caves underground, each about 20 meters wide, 300 meters long, and 40 meters high. Together, they will reach a total volume of 1.1 million cubic meters.

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Water at 140°C will be below ground at more than 100 meters deep

One of the most impressive aspects of the project is the water temperature. The system is designed to store water at up to 140°C at great depth, with enough pressure to maintain operation within the expected conditions.

The caves are located more than 100 meters below the surface, with the deepest area reaching 140 meters. This allows the underground to be transformed into a high-temperature heat storage structure with a strategic function for winter.

Capacity of 90 GWh can sustain a Finnish city for a year

The underground reserve is designed to achieve 90 GWh of thermal capacity. This volume is considered sufficient to heat a medium-sized Finnish city over the course of one year.

In addition to its size, the structure also draws attention for its power. The system’s discharge reaches 200 MW, which reinforces the utility of the project during peak times and enhances the urban network’s responsiveness.

The image shows the **urban heating network of Vantaa** connected to the underground heat storage, which will use **three giant caves** at more than **100 meters deep** to store **1.1 million m³** of water at up to **140°C** and **utilize summer heat in winter**.

Construction began in 2025 and operation is expected in 2030

The main excavation began at the end of 2025 and is expected to progress for about three years. The current forecast points to operational readiness in 2030, following adjustments to the schedule throughout the project’s development.

This timeline shows that the project has moved from being merely a proposal to being firmly in the execution phase. As a result, Finland is beginning to materialize a solution designed to address the seasonality of thermal consumption.

Two electric boilers of 60 MW will raise the water temperature

The project also includes two electric boilers of 60 MW. They will be used to increase the temperature of the stored water and can also send heat directly to the network when electricity is at a more favorable level.

In practice, this enhances the system’s flexibility. Heat can come from either excess available at other times or from the conversion of electricity into useful thermal energy for the city.

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Investment exceeds 200 million euros and changes the logic of winter

The project involves an investment of more than 200 million euros, with more recent projections around 300 million euros. This amount helps to size the scale of an infrastructure designed to operate for decades.

More than just a large-scale excavation, the project repositions the use of the underground as an energy asset. Instead of merely distributing heat when the cold arrives, the city begins to store energy beforehand, with a direct impact on cost, security, and planning.

The initiative transforms heat into a strategic reserve and gives Finland a global showcase in thermal infrastructure. What once seemed merely a seasonal resource now functions as a permanent asset to face winter.

By concentrating volume, depth, and temperature in a single structure, Vantaa creates a new model of underground storage with a real impact on the urban network. It changes the strategic outlook.

Sources: Vantaan Energia / official project, AFRY / project statement

More than 100 meters deep, Finland has begun excavating three giant caves to store 1.1 million cubic meters of water at up to 140°C and to retain summer heat for heating cities in winter.

With water at up to 140°C, three giant caves and a capacity of 90 GWh, the project started in Vantaa places Finland in the race for underground heat solutions and changes the strategic outlook on urban heating

Three caves with 1.1 million m³ will store seasonal heat

Water at 140°C will be below ground at more than 100 meters deep

Capacity of 90 GWh can sustain a Finnish city for a year

Construction began in 2025 and operation is expected in 2030

Two electric boilers of 60 MW will raise the water temperature

Investment exceeds 200 million euros and changes the logic of winter

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More than 100 meters deep, Finland has begun excavating three giant caves to store 1.1 million cubic meters of water at up to 140°C and to retain summer heat for heating cities in winter.

With water at up to 140°C, three giant caves and a capacity of 90 GWh, the project started in Vantaa places Finland in the race for underground heat solutions and changes the strategic outlook on urban heating

Three caves with 1.1 million m³ will store seasonal heat

Water at 140°C will be below ground at more than 100 meters deep

Capacity of 90 GWh can sustain a Finnish city for a year

Construction began in 2025 and operation is expected in 2030

Two electric boilers of 60 MW will raise the water temperature

Investment exceeds 200 million euros and changes the logic of winter

While an average building in London or New York takes three years to be completed, in China a 26-story tower went up in five days with about 100 workers, and the global modular construction market already moves US$ 95 billion per year.

Australia installs giant concrete structures on the seabed 800 meters from the shore to create an ocean intake for desalination that will draw in saltwater and produce 30 million liters of drinking water per day.

While a traditional school may take several days just to raise walls, in Malawi a 3D-printed school had its structure ready in 18 hours, welcomed children in June, and showed a new solution to the lack of classrooms.

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