What did New Horizons discover about Pluto’s possible underground ocean?

Data from the New Horizons flyby suggests that Pluto may possess a subsurface ocean located above its rocky core and beneath a mantle of water ice.1

This hypothesis stems from observations of Sputnik Planitia, a deep, young basin of nitrogen ice. Because this feature is located on Pluto’s tidal axis—directly opposite its moon Charon—scientists determined that there must be extra mass beneath the basin to account for its position. Researchers believe this gravitational anomaly may be explained by the presence of a subsurface ocean.1