Hey everyone! I've been playing this game for over a decade and I've just registered. I love how Halo CE has made it possible to keep this game fresh even after so long. The basic gameplay stays the same but all these talented modders keep creating beautiful new maps for us to explore. Here are some of the secrets I know:

Map: Tempo
There is a secret room that can be accessed by jumping on top of the pillar to the right of green base, and then grenade jumping against the invisible wall. If you are able to land four tricky crouch jumps, you will reach the very bottom of the map. Here there is a rapid-fire fuel rod gun, rocket launchers, flame-throwers, and some kind of red laser beam weapon. The catch: there is nobody to use them on! You must find a tiny little disguised portal in order to get back to the level, where you can unleash hell on your opponents. Alternatively, you can view all the action from the safety of your secret room through the glass floor.

Map: Sciophobia
At the center courtyard of the spaceship, on the red side, melee the door on the second floor hallway while running forward. You'll find yourself in a tiny dark room; jump at the left corner and you will arrive at the top of the map! You can watch the action unfold under you through the glass, and if you wander to the edge, you can also throw grenades through the windows of the bases. This is a great place to hide the flag if there is a standoff in CTF.

Map: Outpost Rio Classic
I need your help on this one. One time I played and I was able to get up to a secret sniping point behind red base. There were rockets up there too. I haven't been able to find my way back though! Would anyone happen to know where the secret portal is?

GAIGHER maps:
Porn

Jhonlex maps: Rips.

One I remember, one of the early hidden rooms is in:

Gauntlet
http://hce.halomaps.org/index.cfm?fid=356

There is a secret/hidden room only accessible by climbing on one of the tree limbs across the canyon and jumping into/thru the canyon wall.

Cool, any idea where exactly that is? I can't find a video anywhere.

To the other three posters, I have no idea what you're talking about. I made this topic because secrets on custom maps aren't very well-known other than Yoyorast Island and Coldsnap, so I thought it would be good to exchange some knowledge. If you don't have a serious contribution to make, please don't post.

honestly, there 3 maps with hidden porn rooms

https://www.youtube.com/watch?v=7JMrJQBlUx4

bigass used to have a great one XD

I never found that one
D:

Valhalla by slappythepirate has one of my favorite secret rooms of all time.
You gotta use a warthog to climb up the structure by the lake to jump onto a ledge, teleporting you to the room of nyan.

People still haven't found the easter egg in the sector 09 footage.

https://youtu.be/z5AIgADdLMY?t=15m43s :v

Does dennis check for pornz inside submitted maps?

I imagine it would be pretty easy but I'm not going to ruin the opportunity for others by sharing the technique. :D