Mud

After rain, the dirt would turn to mud. (PROSE: The Shadow in the Glass, Loving the Alien) Flooding had the same effect. This would make travel difficult, particularly on unurbanised soil. (PROSE: The Shadow in the Glass) Mud was slippery, (PROSE: The People's Temple) as well as sticky, (TV: "The Escape") to the point where one's shoe could get stuck in it. (AUDIO: The Paradise of Death)

Rigel Seven had an abundance of mud, due to its rich soil and heavy rainfall. The local Rigellans appreciated this. (PROSE: Players)

The core of the planet Androzani Minor was superheated primeval mud. When it approached Androzani Major in orbit, a mud burst occurred, bursts of boiling-hot mud, or as Peri Brown put it: "mud baths for everyone." There were mud barriers in place at the time the Sirius Conglomerate were there. (TV: The Caves of Androzani)

In 3985, Bernice Summerfield and Camarina Lannic's expedition were attacked on Menaxus by mud creatures, but managed to escape with the dream machine. (PROSE: Theatre of War)

"Alien mud" encountered by the Eleventh Doctor, Amy and Rory, consumed anything inorganic. (COMIC: The Moon of Lost Hope)

Genetic sculptor Garg Ardoniquist genetically engineered creatures to make bricks out of mud with their heat vision. (COMIC: Stairway to Heaven)

While defending herself, along with the rest of the TARDIS crew, against the Daleks, Barbara Wright thought to make mud from Susan's shoe, and blinded a Dalek's eyestalk with it. She described it as "very sticky and very nasty". This trick enabled Ian Chesterton to get inside that Dalek, to pretend to be one of them, allowing for their eventual escape. (TV: "The Escape") Later, on the way back into the Dalek City, a swamp's uneven ground alternated between rock and thick mud. (TV: "The Expedition")

Visians, an invisible species, could be observed if covered in mud. When made visible, they appeared "thin, bony, with two long, clawed arms, feet like birds' claws, and a narrow head with a beak." (PROSE: Mission to the Unknown)

The "mud" in the Zona Oscura was actually biomass that would allow lost ideas to exist. (PROSE: Party Kill Accelerator!)

Behind the scenes

 * According to Inside a Skaro Saucer, one of the features in The Dalek World which, due to not being actual stories, are not considered valid sources on this Wiki, Mireoton was "the Planet of Mud".