A team of scientists have determined that there was indeed a large subsurface ocean on Pluto at one time.
The scientists were able to use seismology by impact, to study the reaction of a planet’s interior structure to stress waves produced by a large asteroid.
Results suggest a large subsurface ocean beneath the icy planetary surface. They also determined that the planet’s core has experienced “substantial alteration from interactions with an ocean above.”
Their simulations that best reproduce the terrain on Pluto show that Pluto may have hosted a nearly 100 foot thick ocean at the time of impact.