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Perhaps it’s not surprising that people from countries with experience holding back the sea see the potential of building an artificial island to store wind energy.

Belgian cabinet member, Johan Vande Lanotte, has introduced a planning proposal for a man-made atoll placed in the North Sea to store energy.

The idea is to place the island a few kilometers off shore near a wind farm, according to Vande Lanotte’s office. When the wind farm produces excess energy for the local electricity grid, such as off-peak times in the overnight hours, the island will store the energy and release it later during peak times.

It would use the oldest and most cost-effective bulk energy storage there is: pumped hydro. During off-peak times, power from the turbines would pump water up 15 meters to a reservoir. To generate electricity during peak times, the water is released to turn a generator, according to a representative.

The Belgian government doesn’t propose building the facility itself and would rely on private industry instead. But there is sufficient interest in energy storage that it should be part of planning exercises and weighed against other activities in the North Sea, the representative said. It would be placed three kilometers offshore and be 2.4 kilometers wide, according to a drawing provided by Vande Lanotte’s office.

The plan underscores some of the challenges associated with energy storage for the electricity grid. Pumped hydro, which contributes a significant portion of energy supply in certain countries, is by far the cheapest form of multi-hour energy storage. It costs about $100 per kilowatt-hour, a fraction of batteries, flywheels, and other methods, according to the Electricity Storage Association. (See a cost comparison chart here.) And grid storage is a considered critical to using intermittent solar and wind power more widely.

The idea of using an “energy atoll” may seen outlandish on the surface, but it’s really not, says Haresh Kamath, program manager for energy storage at the Electric Power Research Institute (EPRI). This approach, first proposed by a Dutch company, uses cheap materials—water and dirt. On the other hand, the engineering challenges of building in the ocean and technical issues, such as using salt water with a generator, are significant.

“It’s not totally crazy—it’s within the realm of reason. The question isn’t whether we can do it,” he says. “It’s whether it makes sense and that’s the thing that needs further studies and understanding.”