Porta

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

Trivia

 * The Porta will only open for the Master Chief and Covenant Elites, not Pure or Infection Form Flood.
 * The "Porta" was recently parodied in VG cats #240, "...gross"
 * There is also one Porta located on the Halo 3 Multiplayer map, Isolation.
 * Portas make a unique sound when they open or close.
 * 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.
 * Portas are similar to human heart's valve. Any significance of this is up to an individual's interpretation.
 * Because flood forms can't open portas, many players run past the waves of flood on higher difficulties.