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 acts as the Flood analog to doorways. Portas were sighted in the Flood-infested High Charity and in the Halo 3 Multiplayer map Isolation. .

Trivia

 * The porta will only open for the Master Chief and Covenant Elites, not Pure Forms or Infection Forms. It is unknown why this happens. 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 would be submitted to the service of the Flood. However, if the Master Chief came, the porta would immediately know the "threat" as none Flood form and not open, so this theory is probably not true. It probably works like a "sensor" then.