The purpose of this investigation is to assess the possible underground extent of a sinkhole in Colossal Cave Mountain Park southeast of Tucson near Vail, Arizona (Figure 1). The sinkhole is located in a low terrace adjacent to La Posta Quemada (LPQ) Wash. In this report the sinkhole is referred to as LPQ sink. LPQ sink has grown in volume and extent since its initial appearance in the fall of 1994 and has undermined a park road used to access La Selvilla campground and picnic area. Park visitors must now detour around the sinkhole into LPQ Wash in order to access the campground. Colossal Cave Mountain Park is a Pima County park currently operated by Ortega National Parks LLC. Fencing and signage prohibiting entrance was installed around LPQ sink to prevent injury to park visitors unaware of the unstable nature of the sinkhole and surrounding area. Unfortunately, LPQ sink has continued to grow in size over the past several years and now extends beyond the fenced area. The Pima County department of Finance and Risk Management contracted with the Arizona Geological Survey (AZGS) in the fall of 2017 to assess the extent of underground voids associated with LPQ sink in order to predict the limits of possible surface collapse. To best delineate the extent of underground voids AZGS employed a combination of surface mapping, detailed surface topography surveys, and geophysical surveys including microgravity and electrical resistivity void mapping.