David Farley’s new book guides you on a journey to discover a plethora of the most fascinating products of human ingenuity found in varying depths below the Earth’s surface.

Underground Worlds: A Guide to Spectacular Subterranean Places covers a vast timeline of subterranean dwellings, beginning with a section on the Ngwenya mine in Swaziland, which dates back to 43,000 BCE, as well as including a multitude of more modern sites such as various subway systems and the G-Can Tunnels in Tokyo.

According to Farley, humans have always been enthralled by the underground for one reason or another, and he hopes to shed some light on the results of this peculiar obsession. Here is an exclusive excerpt from his book, which details the sheer magnitude of the G-Can Tunnels found beneath Tokyo, Japan:

Excerpt from the Book, Underground Worlds

“There are many superlatives when it comes to Japan. The country is home to the oldest wooden building in the world. Tokyo is the most populous city on the planet. And the Sky Tree in Tokyo is the world’s tallest tower.

And so on the outskirts of Tokyo, lurking beneath a grassy field is another Japanese Superlative: the world’s largest sewer system—albeit, one that flushes out water, in general, and not human waste-tainted water. The Metropolitan Area Outer Underground Discharge Channel, or Shutoken Gaikaku Hosutro in Japanese, is known by non-Japanese speakers as G-Cans for short.

The impressive system is located just twenty miles from the center of Tokyo. Open up the door to a nondescript building in the middle of a field and descend a long set of stairs. Eventually, you’ll be deposited into a massive tank ringed by a forest of fifty-nine thick concrete pillars. The columns themselves weigh five hundred tons each and are six feet wide and sixty feet tall. Five stories high and as long and wide as a football field, this water shaft—known as the Underground Temple—is the main depository for water. But that’s not all.

There’s also a four-mile-long tunnel that runs underneath the ground connecting five different below-the-surface tanks. The massive shafts, which also collect floodwater, are 230 feet deep. And the whole thing is powered by four turbines, the same engines that run a 737 jet airliner. When operating, the engines have the astonishing capacity to drain an eighty-foot-deep basin in one second.

The nearly $3 billion project, which began in 1992 and finished seventeen years later, helps protect thirteen million people in a part of Tokyo that is prone to flooding. One of the unique geographical characteristics of Japan is that 75 percent of the land is mountainous. This means that there are many rivers that flow down to the sea. And in times of heavy rain, those rivers tend to overflow.

If this swath of northern Tokyo gets at least twenty-one inches of rain within a continuous three-day period, then rivers of the area, particularly the Arakawa, overflow and could flood the region. And it’s even worse when one of the regular massive storms hits the city. One famous storm in 1910 ruined 4.2 percent of Japan’s GDP. Throughout the years, these storms have pounded the city, leaving some neighborhoods completely submerged underwater.

After six deadly floods occurred throughout the 1980’s, including one storm in 1991 that flooded over thirty thousand homes (and resulted in fifty-two deaths) in northern Tokyo, city officials realized they had to do something drastic. The solution was the G-Cans.

Here’s how the ingenious system works: Each of the five silos, located about a mile from one another, is near a river. When one of the rivers overflows—often during the monsoon season, from the beginning of June to mid-July—the extra water runs into the tanks. It’s then channeled through the underground tunnels, which are about 30 feet in diameter and located 160 feet underground, to the “Temple”, where it sits until it is eventually drained.

Four Boeing 737-jet-engine-powered turbines pump into six underground tunnels—each about the length of a typical train—that carry the water into the Edo river, where it then easily flow into the sea, causing no harm to anyone.

The turbine engines have the capability of pumping up to two hundred tons of water per second. So is this pricey project worth it? In the first five years of its existence, it was used seventy times.

The main tank, the massive space with the pillars, has been featured on numerous international TV shows and movies, including on Dutch and Australian TV. So, perhaps for some visitors, it is déjà vu.

There are free ninety-minute tours—unfortunately only in Japanese—for those curious about wandering around in the world’s largest sewer. Non-Japanese-speaking visitors must have a Japanese speaker accompany them to explain the safety procedures.

Before descending into the tanks, guests are given an overview, including an explanation of how the system works and why the G-Cans was necessary.

The guide shows satellite photos of Tokyo and points out the various rivers that tend to frequently flood. And after a visit to the project control room with its twenty television monitors and a brief stop on the roof to get a nice view of the nearby Edo River (if you squint you can see the Tokyo Sky Tree in the distance), it’s time to head down to the tanks and one of the most brilliant water discharge channels on the planet, which lies unassumingly beneath a skateboard park and a soccer field.

In times when the system is in use, the tours are obviously canceled.”

David Farley is a New York-based food and travel writer whose work appears in the New York Times, the Wall Street Journal, AFAR, and the Guardian, among other publications. He’s the author of the travel memoir An Irreverent Curiosity: In Search of the Church’s Strangest Relic in Italy’s Oddest Town, which was made into a documentary by National Geographic. He has lived in Prague, Rome, and Berlin and has taught writing at New York University.

Underground Worlds: A Guide to Spectacular Subterranean Places