The hunt for heat: Drilling the deepest holes on Earth

Beneath our feet is an almost limitless source of energy, but while a few lucky locations have geothermal heat close to the surface, the rest of the world will need to dig a lot deeper. The challenge is how to get deep enough.

There are some spots around the world where energy literally bubbles to the surface. In Iceland, home to more than 200 volcanoes and dozens of natural hot springs, tapping into this energy isn't hard. Dotted around the country are steaming pools of water, heated by the geothermal fires that burn just below the crust. Boiling jets of water and steam are thrown into the air by geysers.

The challenge is tapping into that energy.

For other parts of the world to enjoy a part of this geothermal bonanza of clean energy, we need to drill deeper to reach the temperatures needed to generate electricity or provide large-scale heating for nearby neighbourhoods.

With this in mind, some pioneering researchers and companies are turning to new types of drills and drilling techniques to bore some of the deepest holes ever created in the hope of bringing geothermal energy to parts of the world that never thought it was possible.

Quaise Energy, a spin-off from the Massachusetts Institute of Technology (MIT), for example, are aiming to drill holes as deep as 12 miles (20km) to access temperatures of 500C (932F) or more. To do so, they are turning to a tool that draws on years of research into nuclear fusion power. "While others are putting shovels in the ground, we're putting microwaves in the ground for the first time," says the company's co-founder Matt Houde.

"Millimetre-wave drilling is a process that can operate largely independent of depth," says Houde. "And millimetre-wave energy can also transmit through dirty, dusty environments."

But transferring the millimetre-wave drilling technology from the laboratory to a full-scale drilling operation will still be a challenge.

"They have never been used before in the deep high-pressure subsurface environment," says Woskow. "Changes due to intense energy-matter interaction applied to drilling require a new learning curve."

GA Drilling has also been collaborated with Konstantina Vogiatzaki, associate professor of engineering science at the University of Oxford to adapt advanced mathematics looking at how supercritical fluids can be controlled when tapping deep earth energy sources accessed via plasma drilling. "We worked on defining the optimum combustion system for a full-scale drilling tool, opening new horizons in controlling ultra-high pressure combustion through plasma drilling," says Vogiatzaki.

"Our technology is looking to drill up to 11km (6.8 miles) in the future," says geologist and Eavor co-founder Jeanine Vany. "I believe we can make meaningful progress towards unlocking superhot rock in the next three to five years."

Vany also points out that deep geothermal energy doesn't need a lot of space on the surface, which means it could be slotted into urban locations in the future.

There would be a certain poetry in that switch – a power station that once ran on a dirty fuel dug out of the ground finding new life in the clean energy revolution with an energy source from even deeper underground.

The question is – will they be able to dig deep enough?

* An earlier verison of this article stated the Quaise Energy had hoped to convert an old coal power plant into a geothermal plant. The BBC would like to clarify that this was an idea put forward by Paul Woskov and is not something Quaise Energy are currently working on.