Berlin Geothermal Energy: Can Ancient Shell Limestone Heat the City?

Five hundred meters beneath the streets of Berlin lies a layer of rock that could one day warm thousands of homes. It is not coal, gas, or oil — it is porous limestone, soaked with naturally warm water, sitting quietly under the German capital. A research project is now testing whether this hidden reservoir can become a serious source of geothermal energy for the city.

The idea sounds futuristic, but the geology is ancient: the rock formed around 240 million years ago at the bottom of a shallow sea. And the researchers have an unexpected advantage — abandoned natural gas wells that already reach down through the limestone, offering a ready-made window into the deep.

What Is Geothermal Energy — and Why Cities Want It

Geothermal energy is heat drawn from beneath the Earth's surface. Unlike solar or wind power, it does not depend on the weather or the time of day. Deep underground, temperatures rise steadily with depth, and in many regions, naturally occurring warm water or hot rock can be tapped to supply heating — or, at higher temperatures, electricity.

Germany has no volcanoes or geysers, but it does have usable geothermal resources. Warm deep water and heated rock layers exist in several regions, and heat pumps or geothermal probes can already exploit even modest temperature differences between the subsurface and the surface. For cities trying to cut carbon emissions from heating — which accounts for a large share of urban energy use — geothermal offers a steady, local, low-emission option.

Berlin, a metropolis of nearly four million people, heats mostly with fossil fuels. Finding a local renewable heat source beneath the city itself would be a major step toward the city's climate goals.

The Rock Beneath Berlin: 240-Million-Year-Old Shell Limestone

The potential reservoir sits under the Grunewald forest in southwest Berlin, about 500 meters down. It is a layer of shell limestone — known to geologists as Muschelkalk — with a temperature of around 32 degrees Celsius.

What makes this particular rock interesting is its structure. The layer includes two bands of especially porous "foam limestone," each about 15 meters thick. This spongy carbonate rock is full of tiny water-filled cavities, and it holds a substantial volume of warm deep water.

How the foam limestone formed

Around 240 million years ago, during the Triassic period, much of what is now central Europe lay beneath a shallow sea. On the seabed, lime accumulated around grains of sand, shell fragments, and other debris. Over millions of years, many of the enclosed particles weathered away, leaving behind water-filled pores — creating a rock that is both hard and remarkably spongy.

Because of its countless pores and the warm water stored inside them, this foam limestone could, in principle, work as a natural heat exchanger: pump the warm water up, extract its thermal energy, and return the cooled water to the rock.

A Lucky Break: Old Natural Gas Wells as Ready-Made Access

Whether the concept works in practice depends on one critical question: how much water is actually in the rock, and how easily can it flow through the natural fissures? Only if the warm water can be pumped upward in useful quantities can the heat be recovered economically.

Normally, answering that question would require drilling an expensive new borehole straight into the limestone. As Guido Blöcher of the GFZ German Research Centre for Geosciences in Potsdam has explained, such drilling is complex and costly.

But the Berlin researchers caught a break. Directly beneath the foam limestone layer sits a former natural gas storage facility — once used to balance the city's fluctuating gas demand — reaching about 1,000 meters deep. The numerous boreholes drilled for that storage cross the shell limestone on their way down, and they remain in place even though the facility was shut down in 2017. For scientists, these old wells are a unique, low-cost window into the potential heat reservoir.

Inside the ATES iQ Test Project

The investigation is running under the name of the ATES iQ project, and it proceeds in careful stages — because pumping heat from rock is only worthwhile if the geology cooperates.

Mapping permeable layers

First, researchers lower a measuring cable into one of the old boreholes. The cable records temperature along the entire length of the well, showing how temperatures shift when water is pumped up or when surface water is pressed down. As GFZ researcher Jan Hennings explains, this reveals exactly where the permeable, water-bearing rock layers lie along the borehole.

Testing the water chemistry

In a second step, scientists bring deep-water samples to the surface and analyze their chemical composition. The dissolved substances reveal where the water comes from — and, crucially, how corrosive it will be. Salt content determines how aggressively the water would attack pumps, pipes, and heat exchangers during long-term operation. A geothermally promising reservoir can still fail on chemistry.

The nitrogen pump tests

To estimate how much water the limestone can actually deliver, the team runs flow tests using pressurized nitrogen. In one test, nitrogen is pumped into the borehole under pressure, forcing deep water upward so researchers can measure how many cubic meters rise per hour. In a complementary test, water is pushed about a hundred meters down the borehole with nitrogen, which is then released rapidly through a valve — the speed at which the water level rebounds reveals the rock's permeability.

From these rise rates, Blöcher and his colleagues can calculate how much water could later b

e pumped per hour — and from that, whether heat extraction at this site would be economically viable.

Could It Actually Heat Berlin?

If the tests come back positive, the implications go beyond a single well. The porous shell limestone layer does not stop at Berlin's city limits — it continues into the surrounding region. A successful pilot could therefore supply not just parts of the metropolis but neighboring communities as well.

Geothermal heat of this kind would most likely feed into district heating networks or supply large buildings, offsetting gas-fired heating. Because the water is only around 32 degrees, it is best suited to direct heating use (possibly boosted by heat pumps) rather than electricity generation, which requires much higher temperatures.

The honest answer, though, is that the project is still at the investigation stage. Suitability, flow rates, water chemistry, and economics all have to check out before anyone talks about commercial deployment. Research projects like ATES iQ exist precisely to answer these questions before money is spent on full-scale development.

Geothermal Energy in Germany: The Bigger Picture

Berlin is not alone. Germany already operates geothermal heating plants, most notably in the Munich region of Bavaria, where deeper, hotter aquifers supply both heat and some electricity. The North German Basin — which includes Berlin — has long been considered to have geothermal potential, but its moderate temperatures make it a heating rather than power-generation play.

Across Europe, interest in urban geothermal heating is growing as cities look for alternatives to natural gas. Projects that reuse existing boreholes, as in Berlin, are especially attractive because drilling is typically the single most expensive part of any geothermal development. If old gas wells can be repurposed as geothermal access points, the economics of many marginal sites could improve dramatically.

Challenges That Remain

Even with promising geology and free boreholes, several hurdles stand between the test project and a working geothermal supply:

• Flow rates: The limestone must yield enough water per hour to make pumping worthwhile. Low permeability would kill the economics.
• Corrosion and scaling: Mineral-rich deep water can damage equipment or clog wells with deposits over time.
• Long-term sustainability: Extracting heat faster than the rock recharges thermally would gradually cool the reservoir. Careful modeling of heat flows is required.
• Upfront investment: Even reusing old wells, building the surface plant, heat exchangers, and grid connections costs money that must be recovered through heat sales.

None of these are deal-breakers in principle — they are exactly the engineering questions the ATES iQ tests are designed to answer.

Key Takeaways

• A porous, 240-million-year-old shell limestone layer sits ~500 meters beneath Berlin, holding water at around 32°C.
• The ATES iQ research project is testing whether this rock can supply geothermal heat, using abandoned natural gas wells as ready-made access.
• Tests measure temperature profiles, water chemistry, and flow rates to judge technical and economic viability.
• If successful, the reservoir could feed district heating in Berlin and the surrounding region — but the project is still in the investigation phase.
• Reusing old boreholes could make urban geothermal projects far cheaper across Germany and Europe.