Mauna Loa Solar Observatory (MLSO) is a solar observatory located on the slopes of Mauna Loa on the island of Hawaii in the U.S. state of Hawaii. It is operated by the High Altitude Observatory (HAO), a laboratory within the National Center for Atmospheric Research (NCAR). The MLSO sits on property managed by the Mauna Loa Observatory (MLO), which is part of the U.S. Department of Commerce National Oceanic and Atmospheric Administration (NOAA). MLSO was built in 1965.