 Experimentation with a much more flexible grid structure resulted in the release of MODFLOW-USG (version 1.3.00, December 1, 2015), designed to be adapted to a wide range of grid variations using unstructured grids.MODFLOW-USG was replaced with MODFLOW 6, which provided grid capabilities with and intermediate level of flexibility.MODFLOW 6 (MF6), released in 2017, is the sixth core version of MODFLOW to be released by the USGS.