This is the approved USGS MODFLOW 6.6.1 release.
Langevin, C.D., Hughes, J.D., Provost, A.M., Russcher, M.J., Niswonger, R.G., Panday, Sorab, Merrick, Damian, Morway, E.D., Reno, M.J., Bonelli, W.P., Boyce, S.E., and Banta, E.R., 2025, MODFLOW 6 Modular Hydrologic Model version 6.6.1: U.S. Geological Survey Software Release, 10 February 2025, https://doi.org/10.5066/P9FL1JCC
Visit the USGS "MODFLOW and Related Programs" site for information on MODFLOW 6 and related software: https://doi.org/10.5066/F76Q1VQV