Time portals are doorways in time, employed in various fiction genres, especially science fiction and fantasy, to transport characters to the past or future. They differ from time machines in being a permanent or semi-permanent fixture linking specific points in time, and thus are an especially useful plot device when the plot involves characters moving many times back and forth. Appearance Time portals can be represented as vortices of energy, which allow matter to travel from one point in time to another by passing through the portal, or may appear as an ordinary doorway. In popular culture Film In most films, time portals are used to take a group of people or a single person back in time. The most popular of all time travels would be traveling back to the time of the dinosaurs. In some films time portals can also be set as a hazard, for example, they can begin to go haywire which makes them accidentally transport something or someone from a different time to the present; again the most popular is a dinosaur being accidentally brought into the present world. Television The 1966-67 TV series The Time Tunnel. In the 1967 Star Trek episode "The City on the Edge of Forever", a temporarily deranged Dr. Leonard McCoy runs into a time portal, an ancient sentient stone-like ring which calls itself the Guardian of Forever, and is transported back to 1930s Depression-era Earth and history is changed as shown when the Enterprise disappears from orbit. Captain James T. Kirk and Spock follow through the time portal to set things right. In the 1969 Star Trek episode "All Our Yesterdays", Captain James T. Kirk accidentally passes through a time portal. Spock and Leonard McCoy try to recover Kirk. All three are transported into the past of the planet Sarpeidon In the TV series Mirror, Mirror and its sequel Mirror, Mirror II, the story revolves around an old mirror. At the beginning of the story it is discovered that the mirror can be used to travel back into the past, 1919 and 1867 respectively. In the documentary Prehistoric Park in which Nigel Marven activates a time portal to travel to different geologic time periods to rescue rare prehistoric species. In the BBC television series; Goodnight Sweetheart, the main character; Gary Sparrow uses a time-portal to go back in time from the 1990s to World War II era Britain. In