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 soldiers, 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 the highest difficulties.
 * Portas may or may not be the doors on High Charity, covered in Flood spore sacs.
 * Portas have at least two 7 references in them, one being that portas are made up of 7 segments, and that there are 7 portas on the level Cortana
 * The name, porta, comes from the Greek word porta, which means door.
 * It is illogical why, although Portas were created by the flood, it will not open for them.