The the likely location of the rim of the opening that has allowed soft ices of flood out onto the surface of Pluto is marked with broken lines. The actual edge now lies miles beneath the actively light colored and convecting ices of Sputnik Planum. The northern edge of the hole ① is obvious. It appears as the edge of a shadow across the north-western third of Sputnik Planum. The darker color is likely due to the presence of rocks and grit scoured from the submerged surface of the shelf of crust by the soft ice, then carried convectively to the surface.
North of the hole, at the same time as convecting vertically in many cells, the ice of Sputnik Planum (made of mainly of methane, nitrogen and carbon monoxide) is pushing northward over a submerged shelf of crust in a sort of slow motion tsunami. The ice is dirtiest near the leading edge of the northward push because here the ice is shallowest and most freshly scoured.
The crust itself is likely made from a mixture of materials, primarily tar-like hydrocarbons. It is almost certainly not water ice, although water ice may be present as a minor constituent. Pluto’s crust varies in thickness. Before the crust at the location of Sputnik Planum was shattered it was about 5 km thick. Pluto’s crust covers a planet wide ocean of soft “Sputnik Planum ice”.
Fragments of broken crust from this hole now form angular mountain ranges, like al-Idrisi Montes, along Sputnik Planum’s western shore. They were washed up by there by soft ice flows, a.k.a ice tsunamis.
To the east and to the south the location of the submerged edge of the still intact crust is more difficult to identify because remnant soft ice from recent floods of the eastern and southern lobes of Tombaugh Regio is still flowing back towards the center of Sputnik Planum. This returning ice is cold (i.e. colder than the ice in regions where convection is continuing). Any dark material that could reveal the location of the edge has been gathered into clumps long ago due to the convective motions.
The yellow line ② is an estimated location based on the edge of the zone of convection. This assumes that there is little transport of warm ice away from the hole across, the surface of the sunken plate. This might be the case if the edge of the intact plate is at a shallow depth here. The orange line ③ is an alternate possibility that traces a subtle reduction in the size of convection cells, indicating less energetic convection. The reduction in convective vigor may because the warm ice coming from its source within the opening does have to travel laterally across the submerged shelf and so has further to travel to reach the surface, just as the ice chilled at the surface has further to go to drain into the hole.
The hole was probably made when an asteroid impact shattered the crust there, although it might also have been the result of volcanic processes triggered by the slow build up of internal heat below the crust. The hole is an opening to a planet-wide ocean of soft methane/nitrogen/carbon monoxide ice.