Isolation

"Isolation" is a map first referenced in the Halo 3 Beta code. It then appeared in a leaked photo of the list of Halo 3 maps. Most recently, IGN made an article describing it. It is a medium sized map, taking place in an enclosed, grassy area. The environment is in a bio-bubble which has contained the flood. Two bunkers act as the bases for the teams, with openings in the sides for shooting out of the bases. They connect into a network of underground tunnels. These tunnels offer close-quarter combat.

IGN: There's an interesting thing that happens when you play Isolation for a while, correct?

Tyson Green: There is, but I don't want to spoil it.

"Isolation is a medium sized map, set in an isolated biosphere which has been seeded with the basic Flood organism."

This makes referance that the flood might come out if you play the level for a long time. This is reinforced by several screenshots which show what looks like flood biomass in small clusters around the map

The map does well for CTF and Team Slayer games, and even Infection, as recommended by Tyson Green. It is similar to Midship due to it's symmetry and openness on the top of the map.