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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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Abandoned house for 15 years disappears in the woods, shocks owner with unrecognizable scenery and is reborn in an intense transformation after almost 90 hours of work in just 10 days.
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In just three hours, a natural stone floor transforms the entrance of the house with an organic effect, immediate drainage, and a sophisticated non-slip finish that doesn’t puddle water, dries quickly, and impresses with the final result.
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‘Interlocking brick’ made of earth arrives in the construction industry with cost reductions of up to 40% on the project.
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Espírito Santo will receive the largest engineering project in its history with the duplication of BR 262, which will have 50 viaducts, 28 bridges, and 2-kilometer tunnels cutting through the most challenging mountainous region of the entire state.
This space will be used to store heat for long periods, allowing thermal energy produced during times of lower usage to be available when the cold sets in. It is this scale that places the project among the most ambitious in the sector.
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.
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.
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
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