|May 29, 2010 Guatemala City Sinkhole (Business Insider)|
On May 29th of this year, the remains of tropical storm Agatha caused record rainfalls of over 14 inches in Guatemala City, backing up sewer systems. The hole was probably caused by a sewer line that backed up and started to leak, and ash from recent volcanic eruptions may have helped clog the pipse. But a leaky sewer won't make a hole like this just anywhere. Guatemala City is built in a steep valley, with bedrock forming a deep-V. The bottom of the V is filled in with hundreds of feet of pumice-fill, which is loose gravel like rock from volcanic eruptions. Most of the city is built on the pumice-fill, which is easily eroded and can wash out down to the bedrock, up to 600 feet below. In fact, this is not the first time this has happened.
While amazing, neither of these holes are technically sinkholes. Sinkholes are formed from natural water erosion of bedrock (limestone or sandstone usually). The Guatemala "Holes" are not formed by natural water erosion, nor are they formed in bedrock. Unfortunately, geologists couldn't come up with a better term than "piping feature", so sinkhole is probably going to stick.