On July 14, 2015, NASA’s New Horizons flew by Pluto.

Pluto’s atmosphere, as imaged by New Horizons when it flew into the distant world’s eclipse shadow. The atmospheric hazes are clearly visible, but the highest-resolution images were taken on the opposite, sun-facing side of Pluto. (NASA / JHUAPL / New Horizons / LORRI)

At a resolution of only 80 meters (260 feet) per pixel, Pluto was revealed at resolutions thousands of times better than Hubble.

Old, cratered terrain of the Plutonian highlands shows regions that have not changed much over long periods of time. (NASA/JHUAPL/SwRI)

Near the poles, we found cratered highlands: an old, level, icy surface.

The cratered region gives way to a hilly, scarred terrain in a transition region before we arrive at the Plutonian mountains. This is likely the start of a crater wall due to an older, massive impact.(NASA/JHUAPL/SwRI)

That terrain gives way, towards the equator, to hilly, ice-covered regions with scarred markings.

The mountains on Pluto, although spectacular, represent only a very small portion of this icy world’s surface, and are found around the rim of Sputnik Planitia. (NASA/JHUAPL/SwRI)

Hills transition into mountains of ice, some of which rise more than a mile (1600 meters) high.

The edge of the cellular region of the plains on Pluto, with the water-ice mountains just visible off the edge. These mountains only exist for a short while before giving way to a basin rim. This image makes use of additional data from the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). (NASA/JHUAPL/SwRI)

These mountains aren’t static and stable, but rather are temporary water-ice mountains atop a volatile, nitrogen sea.

The geologic structure beneath the surface of Sputnik Planitia. On Pluto, it is possible that the thinned crust is overlying a liquid water ocean. (James T. Keane)

The evidence for this comes from multiple independent observations.

Frozen nitrogen in the mountains, at right, drains through the 2- to 5-mile (3- to 8- kilometer) wide valleys indicated by the red arrows, with the extent of the pooling lake shown by the blue arrows. (NASA/JHUAPL/SwRI)

The mountains only appear between the hilly highlands, after the edge of a basin rim, and young plains with flowing canals.

The basin rim of Sputnik Planitia shows a clear division between the Plutonian highlands and the interior, volatile-rich sea of nitrogen and methane ices. The large mountains exist just interior to the basin rim, which is a crater wall, where the canals “flow” to the inside of the crater.(NASA/JHUAPL/SwRI)

These young plains occur in Pluto’s heart-shaped lobe, which itself was caused by an enormous impact crater.

Sputnik Planitia formed by a comet impact, oriented northwest of its present location, and reoriented to its present location as the basin filled with volatile ices. (James T. Keane)

Only a subsurface, liquid water ocean beneath the crust could cause the uplift we then see, leaving the nitrogen to fill it in.

A high-resolution view of Pluto’s surface close up, including a large portion of Sputnik Planum, the heart-shaped bright, icy region. (NASA/JHUAPL/SwRI)

The observed gravitational anomaly under Sputnik Planitia further indicates a sub-surface ocean.

The geological features and scientific data observed and taken by New Horizons indicate a subsurface ocean beneath Pluto’s surface, encircling the entire planet. (James T. Keane)

Over time, this crater loads with volatile ices, eventually causing the whole world to tip over.

Sputnik Planitia, the left (and only) lobe of Pluto’s famous ‘heart,’ is loaded with volatile ices. This crater is less dense and is the largest deformation on the world in question. It is shown, to scale, with Pluto and Charon accurately aligned as illustrated.

The most frozen, distant known worlds are still active today.