What's unique about the virtual currency bitcoin, besides being relatively new, is that it is owned by no one. There's no government issuer, no bank monitoring, no transaction fees, no restrictions to creating new services, no need, even, to use your real name. Both the system and its eponymous currency exist almost entirely online.

(EPA/LIU XINGZHE/CHINAFILE)

Much of bitcoin's allure comes from its intense level of decentralization. But like any monetary system, bitcoin still requires checks and balances, validation and verification.

That's where "mining" comes in.

Bitcoin users are tasked with keeping their own ledger of every single transaction that occurs in the system, including the ones they're not involved in. To keep everyone's records updated and in sync, thousands of power users — called "miners" — confirm each transaction's validity in real time, essentially by solving complicated algorithms. Miners then send out the verified information to the network.

Mining is a competition, of sorts. Miners use special computer programs to solve the series of codes and functions that make up the highly encrypted transaction data. The first person to get through is rewarded with new bitcoins. Notably, solving the functions relies more on chance and processing power than the miner himself, making it more of a lottery than a math test.

So to be successful, a bitcoin miner needs two things: an array of supercomputers and the massive amount of electricity needed to fuel them.

That's why despite bitcoin's global accessibility, China has emerged as the cryptocurrency's leader. By manufacturing most of the world's mining equipment and building massive mines that take advantage of cheap electricity, China has more bitcoin processing power than any other country, by a huge margin.

(EPA/LIU XINGZHE/CHINAFILE)

The global "capital of bitcoin mining" is China's Sichuan province, thanks to its abundance of hydropower plants and some of the world's cheapest electricity. Bitcoin mines there can gross millions per year — all of it outside of government control.

But because the industry exists in a bit of a legal gray area, the mines are often shrouded in secrecy — locations are concealed and visitors are severely restricted.

That's especially true now that the Chinese government has started to clamp down on bitcoin mines. To get a piece of the action, the state has introduced trading fees and rules to restrain users. Some bitcoin mines have begun to mysteriously shut down or relocate their operations.

But in September 2016, European Press Agency photographer Liu Xingzhe visited a mine built next to an undisclosed power plant deep in Sichuan; the nearest public transportation was almost 20 miles away. The mine had 550 machines running 24 hours a day and was operated by a handful of employees who lived on site and worked on behalf of clients all around the world.

Below, take a tour of this secret bitcoin mine:

The winding road to the bitcoin mines deep in Sichuan province. | (EPA/LIU XINGZHE/CHINAFILE)

A bitcoin mine with a blue roof sits next to a hydropower plant in Sichuan. | (EPA/LIU XINGZHE/CHINAFILE)

Goats from a nearby village walk by the mine's cooling fans. | (EPA/LIU XINGZHE/CHINAFILE)

An employee walks through aisles of mining machines. | (EPA/LIU XINGZHE/CHINAFILE)

(EPA/LIU XINGZHE/CHINAFILE)

An employee works on assembling a calculation board. | (EPA/LIU XINGZHE/CHINAFILE)

A miner meets with clients. | (EPA/LIU XINGZHE/CHINAFILE)

Discarded mining machine boxes pile up in one of the mine's dormitory rooms. | (EPA/LIU XINGZHE/CHINAFILE)

A miner sleeps in his dorm at the mine. | (EPA/LIU XINGZHE/CHINAFILE)

Mine employees use their phones to play games and watch TV outside the mine. | (EPA/LIU XINGZHE/CHINAFILE)