Spiral nebula
A spiral nebula was a gaseous, intensely gravitic cloud important to the formulation of new worlds and star systems.
The Doctor, Leela and K9 once encountered such a nebula on "the edge of the cosmos". Although its high gravity was necessary to bring together the requisite matter to form star systems, it was extremely perilous to any ships that might have gotten too near. The Doctor's TARDIS immediately reacted against the proximity of the nebula by sounding alarms. In trying to explain the problem to Leela, the Doctor said that it was "sucking everything around it like a gigantic whirlpool — including us."
One craft that did not escape its gravity well was the P7E. Like a fly caught in sandpaper, it was trapped inside the nebula, unable to get out. Over time, more and more matter collected on the hull of the ship until it found itself effectively at the center of a newborn planet. The R1C nearly suffered the same fate, but the Doctor was able to devise a successful escape strategy. (DW: Underworld)
Behind the scenes
The term "spiral nebula" fell out of use in the real world in the early 20th century, when it was discovered by Edwin Hubble that they weren't "nebulas" within our own galaxy, so much as entirely distinct galaxies. The depiction of the "spiral nebula" as a moving, localized "event" in Underworld is thus scientifically inaccurate.
