Other forms: sinkholes
A sinkhole is a dent or dip in the ground that's caused by a collapsing surface. Most sinkholes result from erosion.
Water is basically what causes sinkholes; certain kinds of rock naturally dissolve over time from being repeatedly saturated, and the result can be a depression or sunken spot in the ground. Some underground caverns and caves are the result of sinkholes. Human-caused collapses are often referred to as sinkholes too. These occur when unused mines collapse, or in urban environments where water or sewer pipes leak and cause concrete above them to crack and sink.