This project documents an extensive sinkhole collapse that followed the Mw6.4 earthquake in Croatia. The sinkholes collapsed in a small area of two villages, where karstic rock underlays thick clayey sediments. Reconnaissance reports a total of 122 new cover collapse sinkholes within a 1.13 km2 area in twelve months following the MW 6.4 earthquake, described in the GEER report Geotechnical Reconnaissance and Engineering Effects of December 29, 2020, M6.4 Petrinja, Croatia Earthquake, and Associated Seismic Sequence. The project contains spatial and temporal information about sinkholes, and geotechnical investigation works from four boreholes. The audience is geotechnical engineers, geologists, seismologists, and natural hazard professionals.