Geologist Sam Bonis said a sinkhole is defined as an area where bedrock has been eaten away by groundwater and the 100-foot deep, 66-foot wide hole that opened up during the weekend in the capital does not qualify, Discovery News reported.
He said the hole is instead a "piping feature" resulting from the pumice fill, ash flows from ancient volcanic eruptions, beneath the city. Bonis said the ash is 600 feet thick in some places.
Bonis said he and other experts were called in when a similar hole opened up just a few blocks away from the current site in 2007.
"Our recommendation was that this could happen again," he said. "When you have water flowing from storm water runoff, a sewage pipe, or any kind of strong flow, it eats away at the loose material. We don't know how long it has to go on before it collapses. But once it starts collapsing, God help us."
The expert said mislabeling the problem as a "sinkhole" could distract from discussion of a solution, which he said would involve finding new ways to deal with runoff and waste water in the city.