|First Appearance:||Sibling Arrivalries|
- "Wait a minute... afterlife? Underground cave!? What if this isn't Heaven! What if it's...noh, it couldn't be. Could it?"
- "Oh, hey Sarge. What're you doin' down here?"
- "It is! Oh, it is."
- —Sarge in Spelunked
Role in PlotEdit
When Donut was crushed by the arriving ship in The Arrival, he was pushed through the ground and landed in the Underground Caves. Sarge was soon buried after his funeral, even though he was still alive, but somehow fell through a hole in his grave and found Donut in these caves. After realizing that Sister was a Blue Team soldier and his own mistake, Simmons, along with Grif, entered the caves to rescue Sarge.
After reuniting the Red Team, Simmons finds Vic Jr. in the caves before the latter escaped, causing Simmons to realize that Vic Jr. has been manipulating both teams. During this time, Doc, Sister, and Junior enter the caves but are captured by Flowers/O'Malley and the Green Alien, who previously interrogated a drugged Grif with Andy. During Omega's final assault, the Red Team and Doc's group left the caves to provide aid in defeating Omega.
- Sarge originally thought the caves were Heaven and then Hell due to the fact he thought he died.
- Despite its major role in Season 5, an explanation is never given as to the nature of the caves, or the Blue and Red bases there.
- In the Halo 2 engine, the Underground Caves are represented by "Waterworks".