The scientists used geophysical methods to find hidden sinkholes and water channels in a park in Illinois. They discovered a 500-meter long water channel underground, connected to cracks in the rock. The rock surface is uneven, and buried sinkholes show up as low conductivity zones. High conductivity areas indicate sinkholes, water channels, and low spots in the land. Some parts of the water channels have sand "volcanoes", suggesting movement of sediment underground. Probing showed that the ground drops near a known sinkhole, possibly due to empty spaces in the soil or rock.