True sinkholes are found on land that overlays a specific type of rock called "carbonate rock". Limestone and dolomite (see Map 15, Limestone and Dolomite Distribution in Pennsylvania from DCNR site) are carbonate rocks. If you know that the rock under your land is of this type, you may experience sinkholes.
If you are on shale, "slate" or "trap" rock or live outside the areas of the karst features (such as the coal regions), you likely do not have a karst-related sinkhole. Instead, you may have a mine subsidence, ground settlement, a leaky pipe, buried organic material (vegetation, tree roots or trash) that has decayed, or a collapsed septic system.
Sinkholes occur naturally as part of the evolution of the karst landscape. But certain human activities make sinkholes happen faster. Movement of water is the key to sinkhole occurrences. (See "What causes a sinkhole?")
If you are in a karst area, these factors may indicate that you have a sinkhole:
- The hole or depression appears after a heavy rain event (especially after a prolonged drought) or in the spring after the ground thaws.
- Location is near an active quarry.
- Water in a stream or pond swirls and forms a vortex like it's going down a drain.
- Hole or depression keeps reappearing after being filled.
- Ground surface is broken and a void space in the soil (or rock) can be seen extending underground.
- Surface water or stormwater runoff is disappearing into the hole.
For more explanation on sinkhole formation, see Sinkholes in Pennsylvania, the DEP Sinkhole Fact Sheet or the DCNR Sinkholes web site.