Spelunking

Spelunking is a glitch in Halo: Combat Evolved level, The Silent Cartographer, that makes it possible to descend deep into the island.

Walkthrough
Play through the level until you reach the security station. At the left hand door leading down into the island, you will find three overshields. Pick one up and head down into the security station. Kill the Hunters and continue past them into the room with the control panel, and then out the back door.

You will be standing on a ledge, and directly in front of you will be a pipe recessed into the wall. If you look down, you will see at regular intervals along this pipe are small platforms. There are also much larger platforms sticking out of the wall, an equal distance between each of the smaller ones.

All you need to do now is walk off the edge. Aim to land on the first platform sticking out of the wall. The first drop goes quite far, and you need an overshield to survive it. After that, move as close to the pipe as you can and drop down onto one of the pipe platforms, then from there onto a wall platform, etc.

Each time you make a drop, you will need to let your shields recharge.

After making it down onto the fifth wall platform, you will notice that there is a sixth platform that stretches across the entire area. You can either drop down to the pipe platform and then jump to the final area, or just drop straight down to the final area. Once down, there is no way back up, and there is nothing special down there that is noticeable, but it is still fun to explore just for the sake of exploring.