Gravity whirlpool

A gravity whirlpool was described by the Fourth Doctor as "Sargasso sea in space". It would entrap and suck in anything that came too close and crush it into a singularity, much like a black hole. (TV: The Horns of Nimon)

Upon discovering that the entire planet Calufrax was in fact a segment of the Key to Time, Romana I warned that they couldn’t move it without creating a gravity whirlpool. (TV: The Pirate Planet)

One was generated by a Skonnon ship. The Fourth Doctor became trapped in it until he escaped by spinning the TARDIS into an asteroid. (TV: The Horns of Nimon)