Sinkholes have never struck Walt Disney World directly, though
the terrain suggests that it could be vulnerable to them. On Sunday,
a sinkhole opened up in a resort compound less than five miles from
the theme park.
Sinkholes start with water and limestone, two things Florida has
in abundance. Limestone dissolves in water, and the more acidic the
water, the faster the limestone gets eaten away. What starts as a
small hole, deep underground, can grow bigger and bigger until it's
a cave you could walk upright in. Flowing ground water keeps
dissolving away the top, sides, and bottom of a limestone cave,
enlarging it in all directions. For people living on the surface
above these caverns, there's no obvious sign that the ground beneath
their feet is being eaten away from below.
Geophysical surveys can peer beneath the surface, but to the
naked eye, everything looks fine. There's no sign that you're
essentially walking on the surface of a hollowed-out eggshell.
Until a tipping point is reached, and the roof of the cave
collapses - caves in, literally - along with everything that had
been precariously perched above.
That's what geologists call "cover collapse." It happens quickly,
though in the case of the sinkhole that opened in Florida late
Sunday night, it took a couple hours, giving residents time to
evacuate from the surrounding buildings before the catastrophic
collapse that lasted only a few seconds. Thanks to the rapid
evacuation, even though the 24-unit building has been written off as
a total loss, no one was hurt. In addition, officials say that the
sinkhole is unlikely to grow any further. That's the one good thing
about cover collapse sinkholes: Once the cavern below has collapsed,
like an underground bubble bursting, it's over.
In addition to immediate damage caused by the cover collapse,
there's a secondary effect caused by the fractured limestone, warns
Boo Hyun Nam, an engineer at the University of Central Florida.
"Gasoline, heavy metals, organic compounds - whatever soluble toxic
contaminants you had at the surface can, if there's a sinkhole, slip
through cracks and reach the aquifer and damage drinking water
Dr. Nam, who is looking for a way to predict sinkholes, describes
cover collapse sinkholes this way: "We have a lot of voids (holes)
in limestone. Then, on top of the limestone, we have a clay layer or
a sand layer. If we have a clay layer, good. Clay has some
cohesion," which gives it some strength to hold against the weight
of overlying soil, roads, buildings, etc. "It will hold it, hold it,
and then, all of the sudden, it cannot hold it, and BOOM, it
collapses. That's a cover collapse."
The other type of sinkhole, more common in eastern Florida, is
caused by "cover subsidence," and is common where sand sits atop the