General comments:

Whoa! Look at that elevation. You are now 3276.8m below ground. That's just over 2 miles deep. No time to ponder your location however since you're facing a hoard of Headless in a rather small room (see map).

Once you've killed the last Headless a teleporter will appear at the far end (see map) allowing you to proceed to the next stage. No matter how hard you try you cannot harm the red guy swirling in a smokey cloud at this stage. According to the Official Hint Book this is the Demon or "Flaming Smokey Dude". Friedrich told us about the Demon when we asked about the "glass vial" Muller was searching for:

He wouldn't want me to tell you this, but he
was convinced that it contained the essence
of an imprisoned demon.

and...

If it was ever released, it would cause
destruction on a massive scale. Muller
wanted to use it for the war, of course. It
was all nonsense.

Javier obviously released it when he uncorked the vial on Where Only Fools Dare Tread:

Uncorking that vial was the biggest mistake
I ever made in my life.

and...

The old man said it contained the trapped
spirit of some powerful being. I didn't know
how right he was.

and...

I know something appeared, when I opened
the bottle. But I didn't know what it was. I
hardly got a glimpse of it

Note that you can't save you game down here nor can you recharge your health by resting. A Headless will reappear every few minutes disturbing you. It's all the way to end or nothing baby... Ok, Who Else Wants Some?

Second Wave... Zombies

Third Wave... Phantasms  the Violet Crystal comes in handy here.

Fourth Wave... Ghouls

Fifth Wave... Nightmares

Sixth Wave... Oozes

Seventh Wave... Invisible Wraiths  remember to wear the Infra-Red Goggles to see them. ;)

Eighth Wave... Electrical Spheres

Ninth Wave... Skitters

Tenth Wave... Ghasts  shake and bake time!

Eleventh Wave... Venomous Skitters  you'll need a Clear Blue Potion to cure yourself if you get poisoned.

Twelfth Wave... Greater Nightmares  remember those projectiles are guided.

Jason Jones offers some advice  on how to survive these waves of monsters.

If you survive the last of the Greater Nightmares you'll note that the exit teleporter doesn't appear. It's payback time... just you and the Demon. The Demon is in fact quite defenseless at this point one shot (or knife attack) and it folds away in a swirl of clouds. The exit teleporter now appears and the Demon leaves behind the Alien Gemstone mentioned by Javier:

The German's notes said that the only way
to open the exit at the base of the pyramid
was with an alien gemstone.

Since you're going to need it to get out of the pyramid you best pick it up... right? When you do however you'll notice that your health starts dropping rapidly. What's happening? Examining the Gemstone isn't very enlightening and trying to rest won't help. However this information from Jason Jones would have been good to have. Too bad it isn't in the game. So the Alien Gemstone is radioactive!!!

So your health is dropping fast. You've got three choices, drop the gemstone, die, or find some way of carrying it without losing health. A little judicious experimentation should reveal that placing the Gemstone into the Lead Box protects you from its harmful effects. Just as well you kept that box! Note that the Alien Gemstone is worth 5 points and this increases your total points to 44 and your health units also increase from 24 to 26.

Remember to arm the Nuclear Device! You've come all this way... pity to forget your mission objective. You got the code change off Steven right? He changed the first three digits of the seven digit code:

Thanks for reminding me, I almost forgot.
We changed the first three digits of the code
from '287' to '658' because you were
seperated from us.

Leave enough time to get back out.

Items to find:

Alien Gemstone (worth five points and $54K)

Points to note:

According to Matt Soell (Bungie), in an early draft of the Official Hint Book the Demon or "Flaming Smokey Dude" was called "Deceit". Matt wrote:

The Big Blue Meanie was named "Greed" in the draft. Likewise, the "Giant Purple Mutant Hellbeast or Barney on Steroids" was originally named "Malice" and the "Flaming Smokey Dude" went by the name "Deceit." Jason does not remember if these names were his idea or not, nor does he remember specifically why they were dropped in favor of the more colloquial descriptive names found in the final hint book.

Interesting how the Alien Gemstone has a value of $54K, I wonder who valued it?

Decision to make:

You see a teleporter. Teleporting to I'd Rather Be Surfing: Reprise.