Gravity field

A gravity field from a power source 10 miles beneath the surface held Krop Tor in "geostationary orbit" against the black hole, K37 Gem 5. The field extended out as a gravity funnel which allowed access for the Walker Expedition as a "way in". (TV: The Impossible Planet) It functioned as a prison for the Beast, whose body was also chained up beneath the planet's surface: if the prison was opened, then the gravity field would collapse, causing the orbit and the funnel allowing entrance and exit from the planet to disappear. The Beast claimed to have been imprisoned here before the universe the Walker Expedition were native to was created. The field and funnel were destroyed when the Tenth Doctor destroyed this prison. (TV: The Satan Pit)

Behind the scenes
Though most dialogue in The Impossible Planet and The Satan Pit appears to use "gravity field" and "gravity funnel" interchangeably, in The Impossible Planet, the Tenth Doctor specified the two as distinct concepts, saying, "To generate that gravity field and the funnel, you'd need a power source with an inverted self-extrapolating reflex of six to the power of six every six seconds."