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 Pure Forms or Infection Formss. 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 reference to 7.
 * There is a theory that Portas are living creatures themselves because they automatically open when approached. Though if alive, they are submitted to the service of the Flood. If the Master Chief comes, The porta would immediately know the "Threat" as none Flood form. It probably works "sensors" then.