What do you see when you imagine a zero-carbon future? Electric buses zipping by? Rolling hills covered with solar panels? Offshore wind farms towering over the sea? If batteries are part of your vision, good thinking. But there’s a promising if whimsical, piece of the renewable energy puzzle that might be missing from your mental picture: the world of gravity energy storage.

Generating electricity using gravity is hardly a new concept — think of your classic hydropower plant, which captures the energy of falling water via a turbine. But some hydropower systems don’t just produce energy. A “pumped-storage” hydroelectric plant draws excess energy from the grid and uses it to pump water back up into an elevated reservoir where it can fall again. But who needs water when there are all kinds of things we can slide down a mountain or drop off a cliff? Really, you can use almost any material for gravity energy storage, as long as it’s heavy, cheap, and you can figure out how to transport it up and down a steep slope.

In an article posted in the journal Energy last week, the authors depicted a “Mountain Gravity Energy Storage” system that involves a ski-lift-style cable that carries huge bins of sand up and down a mountain. The sand gets stored in an enormous vessel at the top, and when the grid needs extra energy, it’s sent down the mountain, pulled by the force of gravity, thereby powering an electric generator.

Depending on the amount of sand, the height of the mountain, and the speed of the fall, the authors estimate that it can generate electricity for anywhere from five to 555 days. That’s incredible for such a rudimentary system and could become a simple solution for energy storage until batteries become capable of storing massive amounts of energy without degrading. And we don’t even need to turn our beautiful mountains into eye-sores to make this happen; engineers are already patenting gravity storage systems on the slopes of defunct coal mines.