How Sinkholes Work

Sinkholes look dramatic when you're peering down into one, but it's below the surface where the real action takes place. A sinkhole's identity centers on the gaps, crevices, cavities and voids that lie under the overburden, or the soil above the bedrock. As these gaps develop, expand and merge, soil from the overburden starts filling the void.

In the case of the sinkhole suddenly opening to swallow a car that we just mentioned, it formed because the overburden was no longer stable enough to hold itself up. This common type of sinkhole is known as a collapse or cover-collapse sinkhole. Although these sinkholes can appear suddenly, the erosion that makes them happen has likely been taking place for weeks or years before, underground and out of view. So let's head underground.

The top layer of a cover-collapse sinkhole is usually a soft overburden, made of soil with a lot of clay in it. Over time, a small cavern forms underneath that soil. Opportunistic sediment takes advantage of the newly free real estate and starts spilling into the cavern -- a process known as spalling. As spalling continues, the underground cavern fills with more sediment, hollowing out the space under the overburden. Eventually, the overburden becomes so thin that it suddenly collapses into the cavern below (this is the "cover collapse"), creating a sinkhole. In some cases, the cover can support its own weight but crumbles when additional weight, whether from a hapless person or car, passes over it.

With cover-subsidence sinkholes, water permeates the soft overburden. An example of this type of terrain is an overburden made up of up to 100 feet (30 meters) of sand with a small amount of clay below before yielding to soft limestone. As limestone dissolves and leaves a void, sediment from the overburden seeps in, creating a bowl-like impression in the Earth. These sinkholes, often only a few feet across and deep, are smaller than many others because after reaching a certain size, sand and sediment pour into the hole [source: Southwest Florida Water Management District]. This inflow of sediment can block the outflow of water by stopping up the cracks and passages that connect the sinkhole to underground conduits. Many of these sinkholes then become ponds, as the water has nowhere to drain. They also don't produce the spectacular kind of cave-in associated with a cover-collapse sinkhole.

A dissolution or solution sinkhole is essentially a deep impression in the ground. These sinkholes usually have a thin overburden (if any), which washes away or is eroded. The now-exposed rock then progressively dissolves when it touches acidic water. The bare area hollows out, forming a shallow basin. Water often pools on the surface of a dissolution sinkhole, creating a sort of pond that compounds the erosion of rock below. Also, like in other types of sinkholes, objects and other material can get stuck in the fractures where water would normally flow out. Unlike a cover-subsidence sinkhole, there is no overburden or covering sediment to spall into the sinkhole or cover the eroding rock.

Human activity produces some of the worst sinkholes. We'll take a look at them on the next page.

In urban settings in particular, sinkholes may owe their development to human activity as much as anything else. In some cases, sinkholes appear because of the combination of industrial activity or development, soft limestone bedrock and less than 200 feet (61 meters) of overburden [source: Southwest Florida Water Management District]. The following human actions can also cause sinkholes:

Sinkholes may also form in places where water flows regularly, such as a storm drain, or when water sources are diverted or cut off, especially by pumping out groundwater. A broken pipe can contribute to sinkhole development by flooding and weakening the soil, but it can also provide an outlet for the dirt and rock that's supposed to surround pipes. After enough material falls into pipes and is transported away, the surrounding earth becomes destabilized, contributing to sinkhole formation, sewage or water spills, or other disasters.

­One­ of the worst human-caused sinkholes occurred in 1994 in Polk Cou­nty, located in Central Florida. First, some background: 30 percent of the world's phosphate comes from Florida and often ends up as an ingredient in fertilizer [source: Laufenberg]. That booming phosphate industry comes with tons of mildly radioactive waste known as phosphogypsum. Florida mining companies arrange phosphogypsum into massive stacks that can cover hundreds of acres and weigh millions of tons. Other phosphogypsum disposal methods, including recycl­ing it for use in construction or artificial reefs, have yet to take off because of debate over whether the material is too radioactive to reuse. Almost a billion tons of the stuff is stacked in Central Florida [source: EPA]. All that weight pressing down on the soft, sinkhole-riddled Florida terrain can lead to disaster.

So it was in 1994, when one of these stacks collapsed into a 150-foot (46-meter) deep sinkhole. Millions of tons of phosphogypsum entered the sinkhole and polluted the state's water supply. The cleanup cost millions and cast a light on both the problem of sinkholes and the need to find uses for the millions of tons of byproduct waste produced by the state's phosphate industry.

­We've talked about how sinkholes form and some of the basic hazards involved. But what other problems can they cause? We'll explore that subject in more detail on the next page.

Sinkholes aren't always a grave danger. Sometimes, depending on their size, they're just an inconvenience. It's their sudden or deceptively large appearance that makes sinkholes dangerous, both to human safety and to buildings and infrastructure. Construction projects frequently have to contend with the specter of sinkholes brought on by broken pipes or careless drilling. These geologic cavities can be expensive to repair, voracious in their appetite to swallow homes and cars, and toxic in their ability to contaminate groundwater supplies by sucking up debris or sewage.

­Flooding looms as a particular concern when it comes to sinkholes. We already discussed how flooding contributes to sinkhole development, but sinkholes can also factor in to the flooding of a surrounding area in several ways. If the outlet that connects a sinkhole to a conduit or water supply, such as an underground stream, becomes filled with trash and debris, water has nowhere to flow. The basin or sinkhole then fills with water, causing flooding. Sinkhole flooding can also occur simply because water enters a sinkhole more quickly than it can drain through a conduit or other outlet. The last flooding scenario happens when an overflowing river is connected to a sinkhole via underground streams and conduits. Water flowing through the sinkhole would normally drain into the river. But because the river is already flooded, flow reversal takes place and excess water shoots back into the sinkhole, filling it with water and potentially flooding the surrounding area.

Sinkholes can appear suddenly and with great violence, and they're something that any city planner or engineer must keep in mind. One notable, tragic case of this type occurred in Guatemala's capital, Guatemala City. Early in the morning on Feb. 23, 2007, a 330-foot (101-meter) deep sinkhole appeared almost instantly, killing at least three people and devouring homes in a packed urban area. The ground shook and a terrible smell wafted from the ruptured sewer lines that, along with rain, were thought to have caused the disaster. Although the catastrophe occurred suddenly, residents reported hearing and feeling the ground shaking about a month before the sinkhole appeared [source: Associated Press]. The government had plans to send a robotic camera system to investigate, but it didn't act in time.

Sometimes we don't need a government agent to confirm that a sinkhole honeycombs the area. Rather, it's obvious from the gaping hole in the ground. But other times, sinkholes may be difficult to detect -- for example, if they're still covered with a thin overburden that has yet to collapse into the void below. Property damage, especially around the foundation, may be a subtle sign that something is wrong. In addition, if vegetation is dying unexpectedly, a sinkhole may have disrupted underlying streams that usually feed the plants. Other signs to look for include polluted or muddied well water, formation of new ponds and trees, or sign posts that appear to be slumping over.

Some people use sinkholes as makeshift trash cans when, in fact, anything thrown in them may end up in the local water supply. In many communities, this practice, in addition to being harmful to the environment, is illegal [source: Pennsylvania Department of Environmental Protection].

­If your house suffers damage from a sinkhole, it may be covered by homeowner insurance. Sinkholes generally should be fenced off or otherwise marked. The Southwest Florida Water Management District recommends filling them in with sand, though many homeowners turn to professional contractors [source: Southwest Florida Water Management District]. If they sit on public property, city officials need to be contacted. A severe sinkhole found on public or private land may have to be repaired with concrete or require earthmoving equipment to stymie erosion.

The United States can't afford to ignore sinkholes. The country's aging sewer and water systems have experts worried that deadly sinkholes like Guatemala City's could develop [source: Rooney]. Their worries, no doubt, are founded on the fact that many essential infrastructure improvements across the country have been neglected. Pipes, bridges and the like that were intended to last 50 or so years are approaching or have passed their intended shelf life [sources: Rooney, Wald]. For example, you may remember the disastrous Interstate 35W bridge collapse in Minneapolis that occurred on Aug. 1, 2007, and killed 13 people. Authorities later attributed the bridge collapse to faulty design and the gradual addition of more weight, such as concrete lane dividers, without upgrading the bridge's support systems.

­We already discussed the large sinkholes in Polk County, Fla., Guatemala City and Berez­niki, Russia. On the next page, we'll look at some other unusual sinkholes from around the world.

Perhaps no sinkhole is more distinctive than the Qattara Depression. The massive sinkhole is more like a large basin, stretching across a wide swath of Egypt. It's considered the largest naturally forming sinkhole on Earth [source: WebEcoist]. The depression is "only" 436 feet (133 meters) deep, but it's also 75 miles (120 kilometers) wide and 50 miles (80 kilometers) long. The depression's difficult terrain made it an important defensive border for the British army during World War II, and now it's considered a tourist attraction. Various schemes have been conceived to harness the Qattara Depression for energy production. One possibility calls for filling it with water and then using the basin's low elevation and water evaporation as part of a hydro-solar project.

­Like the Qattara Depression, sinkholes can be beautiful and relatively harmless, though frequently the difference between stunning and disastrous is a matter of location. Venezuela is known for its Sarisarinama sinkholes: basins 1,000 feet (305 meters) wide and at least that deep that are practically perfect circles. These sinkholes are made more distinctive by where they appear; they lie on top of high mesas in the country's tepui region. Tepuis are incredibly isolated areas filled with lush forests and waterfalls. Sarisarinama sinkholes were first discovered in 1974, high up on flat-topped tepui mountains, and have been described as part of a "lost world" [source: Melham]. These sinkholes have been treasure troves for scientists, who remain fascinated by the unique plant and animal species found nowhere else but in some Sarisarinama sinkholes. The sinkholes and the tepuis have also attracted numerous explorers throughout the centuries, some lured by the challenge of their remote setting, others following rumors of massive gold nuggets washing up in streams on top of tepuis.

­If you've ever seen the "Planet Earth" documentary series, then you're probably familiar with cenotes, water-filled sinkholes connected to underwater caves and streams. Cenotes often form gorgeous swimming spots and feature underwater caves, waterfalls and other intriguing formations. The Bimmah Sinkhole in the Middle Eastern nation of Oman is similar. This sinkhole attracts many tourists to its shimmering, translucent blue-green water. Similarly, Mount Gambier in southern Australia is known for its picturesque water-filled sinkholes and volcanic craters. Some of these sinkholes contain well-manicured, terraced gardens.

One of the world's centers of sinkhole activity is the Dead Sea, the salty body of water sandwiched between Israel and Jordan. The Dead Sea is the lowest point on the planet (1,371 feet or 417 meters below sea level) and an important tourist and historical site, as well as part of a migratory route for hundreds of millions of birds. The water level of the Dead Sea has been dropping yearly because of work by the mineral extraction industry and diversion of water from the Jordan River, which feeds the sea, for use in watering crops and providing drinking water.

The newly uncovered areas of the Dead Sea have become riddled with sinkholes. Evaporating freshwater pools are breaking down salt deposits, creating "holes" and destabilizing the ground, leading to sinkholes. Various remedies have been proposed, including multibillion-dollar canals connecting the Dead Sea to the Mediterranean or Red seas. One scientist has theorized that a point of equilibrium will inevitably be reached, as the sea will become too salty to further evaporate [source: Anderson]. But by then, the Dead Sea will be about one-third of its early 21st century size, and in the meantime, resorts around the sea have struggled with receding waters and the unstable, sinkhole-laden earth left behind.

For more information about sinkholes and other related topics, please explore the links that follow. ­