It can be scary when the ground suddenly opens up, like it did in South Amboy when a sinkhole swallowed a car early Tuesday morning. Experts say there are two main types of sinkholes and those have been occurring in New Jersey lately are, in a sense, man-made.

According to Ramapo College Geologist Emma Rainforth, the man-made sinkholes are the result of things like water main failures or construction shifting the soil.

"Naturally-occuring sinkholes are not all that common, because we don't have much dissolveable bedrock," Rainforth said.

She said the sinkholes occur occur wherever there is bedrock that is mostly limestone or marble. According to Rainforth, the types of sinkholes that we are seeing a lot in New Jersey at the moment, are man-made.

"The water main breaks -- it causes erosion of the soil that the water main is embedded in,  and so it washes away the soil, leaving just behind the surface, such as a roadway or pavement or backyard," she said.

Greg Herman of the New Jersey Geological and Water Survey within the State Department of Environmental Protection says says naturally occurring sinkholes are more common where there is a lot of limestone in the ground.

"They're very common up in Warren and Sussex county areas in Northern New Jersey where you have a lot of limestone substrate," said Herman, who is also president of the New Jersey Geological Association.

Man-made, or natural, sinkholes unfortunately do not provide much warning before the ground opens up. According to Rainforth, sinkholes around an old water main can occur pretty much at anytime.

"They quite often do occur when you have heavy rainfall, for instance, that can then help with the washing out of the soil, or anytime the water main is able to break," she said.

Rainforth said the message here is not to be alarmist. In the grand scheme of things, when you think about how many water mains there are in the state of New Jersey, and how many of them actually break, it is a  tiny proportion.