Tar pit

According to the Tenth Doctor, tar pits were responsible for humans knowing about extinct animals. Beasts of earlier epochs would become trapped in the tar and die, but the tar would preserve the animal's skeleton more or less intact. Some of the most famous tar pits were the La Brea tar pits in Los Angeles, which the Doctor said had preserved thousands of animals. (NSA: The Last Dodo) There was a tar pit located in London in 1993 on an alternate Earth, when the Seventh Doctor's TARDIS fell through into that universe it became trapped in a tar pit, it remained there until just prior to the Doctor initiating a time ram with his alternate self's TARDIS at which point it was removed by the Fortean Flicker. (NA: Blood Heat)