Porta

Were you looking for Portal, the Forerunner complex? A Porta, is a sphincter-like orifice embedded into the floors and walls of Flood hives that act as the Flood analog to doorways. Portas were sighted in the Flood-infested High Charity and the Halo 3 Multiplayer map Isolation..

Trivia

 * The Porta will only open for the Master Chief and Covenant Elites, not Pures or Infection Forms. Because Flood Forms can't open Portas, many players run past the waves of Flood on higher difficulties.
 * Portas may or may not be the doors on High Charity, covered in Flood spore sacs.
 * Portas are constructed of 7 segments, another 7|7 reference.
 * There is a theory that portas are living creatures themselves because they automatically open when approached.