In a big, scaffolding-riddled room at the Max Planck Institute for Plasma Physics of Greifswald, north east Germany, a bunch of physicists and engineers are getting very excitable.
They're about to turn on a 16-metre-wide machine that has the potential to end the energy crisis.
It's name? The Wendelstein 7-X, a stellarator fusion reactor. This is why it's important.
Don't we already have nuclear power?
Most nuclear reactors are 'fission' reactors - making energy when large uranium atoms decay, splitting into smaller atoms and release energy. This energy is then used to create steam to turn turbines.
It's cheap, easy energy, but the payoff is deadly radioactive waste that takes hundreds of years to become safe.
So what's a fusion reactor?
A fusion reactor does the opposite to a fission reactor, forcing hydrogen atoms together in a process that gives off even more energy - but it's very hard to control.
The few that are working so far only produce a small burst of energy.
The Wendelstein 7-X is a very special kind of fusion reactor, called a stellarator.
A stellar what?
Sure, it's a title befitting the mad experiment of a Marvel professor shortly before they're turned into a super mutant, but it's far cooler than fiction.
Here it is being built - a process that's taken nine years. If it works, it'll create heat in a similar manner to our Sun.
So how does it work?
Fusion reactors have to control the collision of hydrogen atoms to create energy. Most are 'tokamak' reactors, like the one in the previous image: they containing a hot, spinning loop of hydrogen gas (plasma) in a regular doughnut shape created by magnetic and electric fields. They take a massive amount of energy to heat and pressurise the hydrogen, and their energy output is minimal.
A stellarator reactor uses a precise, complicated series of single electromagnetic coils to shape the 'perfect' magnetic field for fusion to take place. It might look like a weird wiggly water slide, but it's incredibly precise.
It's hoped this odd stellarator shape will use a lot less energy to heat plasma up, and be a lot more stable when running.
What's going to happen when they turn it on?
It's hoped the Wendelstein 7-X will run for a maximum of 30 minutes - which might not sound much, but would be a considerable improvement from the fusion reactor record currently held by French “Tore Supra” reactor, at six minutes and 30 seconds.
It should produce a similar amount of energy as other fusion reactor experiments, but it'll use less energy to get up and running.
The Wendelstein 7-X is only an experiment - a massive 'proof of concept'. It's not about to start powering all of Germany with clean, endless energy, but it could pave the way to the power plants that will.
Approval has been given to flip the switch - with the first test set to go ahead at some point this November.
Is it safe?
The conditions for nuclear fusion are incredibly hard to control - but if the Wendelstein 7-X loses stability, the reaction would stop happening rather than becoming all explosive and deadly.