Sinkholes are often formed as a result of the actions

Swallow hole is also known as a sinkhole refers to a hole or depression in the ground resulted due to certain kind of collapse of the surface layer. The majority of them are the result of karst procedures, like the suffosion processes or the chemical dissolution of the carbonate rocks.  

The sinkholes differ in size from 1 to 600 meters, both in depth and diameter, and differ in form from soil-lined bowls to bedrock-edged chasms. The sinkhole may get produced suddenly or slowly, and are observed all through the globe.  

that tool would be a compass

the answer is b. dilution