Mining bitcoin throws off an enormous amount of heat. That’s because the “mining” in question refers to the energy-intensive computational process by which bitcoin transactions are verified. In a typical transaction, a boxy computer attempts to solve what’s essentially a very complex math problem. If it can do this before any of the other “miners” working on the problem across the world, the miner is rewarded with bitcoin of its own.

This process takes a whole lot of power; overall, bitcoin mining accounted for an estimated 0.5 percent of global electricity use in 2024. The more complex the task at hand, the more electricity is needed—and the more heat is created. Essentially, as long as it’s lucrative to mine bitcoin, it’s going to spit out a lot of extra heat as a byproduct. The question becomes: Can that heat be put to beneficial use?

That’s where Carlsson comes in. He’s now helping to heat the homes of 80,000 residents in Finland with waste heat from local cryptocurrency miners, as a part of a project run by his new employer, the bitcoin mining company MARA Holdings.

Water runs through MARA’s miners, which are stored in black metal units in the center of the towns, cooling them off before coming out at a scalding 122 to 172 degrees Fahrenheit (50 to 78 degrees Celsius). From there, the water is pumped underground through the cities’ existing district heating systems, drastically cutting down the need for traditional boilers. As a result, MARA’s two bitcoin districts have avoided greenhouse gas emissions roughly equivalent to those produced by 700 US homes since its first project came online in 2024.