The Falls City underground limestone mine in Clark County, IN, was abandoned in 1903 ± 3 yr. The mine is now flooded and partly collapsed. A map of the mine was needed because undermined areas would be unsafe for buildings. We used this mine as a test site to evaluate gravimetric surveying to map other underground mines in the region, some of which cannot be entered. We first mapped the Falls City Mine using a commercially available "line finder," normally used to locate underground utilities

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... ch as pipes and telephone lines. Part of our crew carried the continuously operating, 82.5-kHz vertical-dipole transmitter through the underground mine in a boat. Aboveground we followed the boat using a receiver which generated an audio pulsing tone with higher frequency indicating greater magnetic field intensity. Gravity was measured at 4-or 8-m intervals in 10 profiles (total length = 2.5 km). In four of these profiles, inflection points of 300-µgal Bouguer gravity anomalies located edges of the uncollapsed portions of the mine. Gravity profiles around the perimeter of the collapsed mine were used to search for any tunnels inaccessible from the surface. None were detected. The gravity method should be practical in mapping similar underground mines in the region.