Flood hive

A Flood Hive or Flood Wall is an important step in the development of the Flood Life-cycle. Once a hive has been established, it serves as a base for Flood assaults as well as housing a flood Gravemind. Not much is known of Flood Hives, as only one example has been observed.

Background
The processes that develop a hive begin during the "coordinated stage" of Flood development, once a viable Gravemind has been developed and the Flood has access to a sizable population. The Gravemind is a collection of the sentience and knowledge that has been gathered by the victims of the Flood Parasite.

Early Stages
In order for a hive to be founded, the Flood must have access to a suitably large populace and an environment capable of sustaining flood life. The population of the area has to contain high enough calcium deposits to sustain the flood parasite.

Once a useful area has been found, the Flood proceed to attack the hosts present, gathering food and biomass. The Flood attack with a strategy relying primarily on numerical superiority. The parasite then begins to pump spores into the atmosphere, restricting breathing for the hosts present, causing them to die of asphyxiation, or weakening them in order to make them less resistant to assimilation. Hosts who are infected will add to the Flood's arsenal. Older combat forms will develop into Carrier Forms, which generate more infection forms, continuing the cycle.

During this time, Flood Growth Pods start to grow on surfaces. Growth pods are constructed from flesh, and grow at an extraordinary rate. It is unknown how they develop, as this has not been observed, but it is reasonable to assume that they grow from spores that root themselves in a spot capable of sustaining them.

Later Stages
Once the Flood has access to enough calcium, they begin to develop a hardened carapace, onto which they add Flood Super Cells to form Pure Forms, a form of independent Flood drone which has no need for a host. As a result of this development, Combat Forms are relegated to defensive positions or are converted into more pure forms.

The functions of objects, such as doors, are replaced as growth pods grow over the original architecture. Under the Gravemind's orders, life support systems are sabotaged in order to speed up the surrender of the inhabitants. The pouch forms eventually consume most or all of the original area. The growth pods grow large, pustule-like growths, which incubate new infection forms, essentially replacing Carrier Forms and further reducing the Flood's dependence on hosts.

Once the process is complete, the Flood disposes of almost all need for new biomass, becoming self-sustaining. Despite this, due to the Flood's natural instinct to infect and consume all life, the parasite rarely becomes content at this stage.

Trivia

 * High Charity is the only known Flood Hive.
 * Flood Gestation pods can be seen all the time, but the only time one can see the underside of a Gestation pod is on the downloadable map Halo 3 Cold Storage
 * Interestingly there seems to be less green mist in the later stages of Flood Hive creation, this is easily noticeable From the Halo 2 level High Charity to Halo 3 level Cortana
 * The only instance of a Porta outside of Campaign is in the Halo 3 level Isolation, suggesting the area was about to become a Hive.
 * It's possible that the the flood area outside of Cold Storage could be part of a Flood Hive.