First off, Trekka. Why the name? I was listening to the Puscifer album the whole time I made this, when Adam Carolla wasn't talking in my ears. By the point of naming it, it just seemed right. Here's the song it's named after, if you were curious.

STORY: The UNSC has for the past 24 years since the defeat of the Covenant secretly had fusion mining facilities installed all over Delta Halo, since scans by ship_vessel_IN_AMBER_CLAD revealed pockets of previously undiscovered elements in certain areas. Extraction Facility T34 (codenamed TREKKA) was built into a large rock structure heavy with "Grapium" and "Jambium", and a large tube transported the elements all across the valleys and oceans to the Collection Department.
The facility gathered elements via a large magnetic device in the center.

However, the rumblings created by the machinery shook the very foundations of the mountain, whose roots went deep into the Ring. There, many leagues below in the dark and fogged hallways and holes, the Flood learned of the operation, and of the food that lay behind it. A Gravemind with little effort can soon learn the goings-on of its home, and this one was no different. It reached out its long tendrils out across the Halo, and found many UNSC sites; mines, factories, outposts.
Organizing its subordinates, it attacked. For the past two days, the UNSC has been fighting a losing battle and requesting evacuation. The navy won't send ships too close to the ring, else they risk the contamination spreading again. Most of the world still held the glassed section of Africa from the War against them.

This is where you come in: you and your team of operators are being attacked by the Flood (the Gravemind has opened a portal that leads directly inside TREKKA, and you must hold out on top of the magnet. One of your peers has keyed in a code to open up a maintenance path to the Element Transport Tube, but it takes a few minutes to unlock completely. You'll have to survive on the magnet for a while.
After the maintenance way reveals itself, you must run for survival!

The last bit of the way takes a little longer, but soon enough it shows up as well. Or at least, most of it...

Luckily, a heat monitor on the wall shows a sliver of hope...

And once your team has made the leap of faith, another is called for. The entrance to the Tube is just below!

Crawling through the thing is dangerous, because the Flood are onto you, right above you, to be exact. But with caution and a lowered head, you can ford your way down the facility.

At the bottom of a drop, you come upon a strange sight:

Bathed in flashing red emergency lights, there are UNSC weapons lying around in here; there must have been a rescue team sent after you, but they too were overwhelmed and consumed by the Parasite. Your team mustn't meet the same fate, so you decide to travel the Tube across the ocean to the nearest outpost and get an escape vehicle.

Only...

the Flood has, by some manner of treachery, destroyed your only escape beyond repair. Support pillars of the Tube now are the only bits left standing, as the remnants have been cast down into the sea:

At first it seems all is lost, but don't abandon all hope yet!

There is one jetpack left from initial facility construction; there isn't a lot of fuel left in it, but there should be enough. You must choose one friend from your group to trust with this equipment. The rest of you must stay and try to hold the line against the zombies while your chosen companion navigates down into the ruins.
The journey across the waves of detritus will be perilous...

...but with perseverance and luck he may reach the other shore and find a ransacked outpost, with a grand gift perched atop it:

If your friends have been resisted, then your escape plan may work! Head over to pick them up.

Only, you can only carry two passengers. Here you must make a decision: who will survive, and who shall be sacrificed? Eventually, you and your chosen survivors set off into the skies to rally with any remaining UNSC forces to coordinate an effective counterstrike. However, the Flood always come in great force, and often have several tricks up their sleeves.....

Tread the skies carefully.

...else they obtain you as an instrument as well.

HAVE FUN! :D

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Added after 1:

is it OK to bump these things if everyone's seemed to have missed it? lol :P