Hydrologic Oddities: The Biggest Dam You'll Never See

While looking for strange rivers, I've come across many that run underground (see posts about the Underground River in the Philippines, in Wyoming, and so on). Are there also underground lakes? There are lakes in caves, but more commonly people get groundwater from aquifers below the water table. But what about underground reservoirs?

Strangely enough, there are several underground artificial lakes on the small island of Miyakojima, Japan. The limestone rocks are highly permeable (i.e. water flows quickly through them) and they have a basement of mudstone, through which nearly no water can pass.

Concept of the underground dam. On the top is the cross section of the island without the artificial dam. How the water table is changed by the dam is shown on the bottom. 

In 1993 the Japanese built artificial walls that go down to the bedrock and hold back the groundwater flow. The underground walls at Sunagawa and Fukuzato hold back 20 million cubic meters of water, about 7 times the volume of the lakes in New York City's Central Park. There are two main dams, one of which is 50 meters (nearly 165 feet) tall. Wells dip into the underground lake to pull water to the surface for irrigation and other uses.

A spot where the lip of the underground dam is visible from the surface. The lip of the dam is to the right of the walkway.

There is a good technical writeup describing the underground dams. There is also a good section in this hydrogeology book.

Overview of the island, how groundwater flows (lines), where it naturally comes to the surface (dots) and where the underground barriers were built (heavy black lines).

The good news is that there's none of the usual negative impacts of above-ground reservoirs, such as having to relocate homes or having the potential for catastrophic failure. In theory, the dams could last for a very long time because they are buried and aren't worn away by the weather. But the bad news is that it is expensive to fix anything underground and it costs extra money to pump the water to the surface.

The above-ground museum for the underground dams

Other underground reservoirs exist, some dating back to the 1800s (but these are usually hollow tanks and are not built to hold back the actual water table). The ones in Japan might not even be the last, either- a proposed reservoir under Beijing is nearly 250 times the volume of Sunagawa and Fukuzato put together.