Many thousands of years ago, the planet was colonized by an unknown, advanced species, likely hoping to take advantage of the plasma deposits and thermal vents. They lived primarily around the cave areas, using the local plasma deposits to build structures and objects. It was them who either created or attracted the xenomorphs, this lonely little world becoming a fierce battleground between the two species. It was the xenomorphs who won the war -- the original aliens disappeared entirely, leaving only their dusty buildings which rapidly began to vanish under the sands.
The Yautja were also attracted to the area, having picked up the signs of uncontrolled xenomorph infestation. They built several temples on the planet's surface, using them as hunting lodges from which to clear out the nearby hives. In fact, they did their jobs a little TOO well -- the xenomorphs fled deep into the planet's bowels, lurking in plasma-carved caves so far that no light would ever reach there, and going into dormancy lest they be wiped out completely. The yautja, satisfied, left the world but kept their own temples in the area, unconcerned with the planet's previous inhabitants or what they might have left behind. Their only concern was hunting.
Roughly a year ago, Weyland-Yutani discovered the planet thanks to their long-range exploratory vessels. The ancient ruins in particular interested them highly, and the rich plasma deposits were a significant bonus. Their Modus Operandi in a place like this, so out of the way, was to send a fleet of pre-constructed, self-sustaining spacefaring buildings -- "Orbital Drop Pods", to the planet and float them by thruster power directly into place on the surface. These included research stations, archaeology labs, and most importantly a set of archaic borehole-drilling geothermal plants that could take advantage of nearby vents to draw power. In addition, they dropped an atmospheric processing plant which could encourage the plant life to flourish once again even in the planet's night cycle (this would unfortunately lead to extreme jungle growth when there was nobody around to maintain them). The Company dropped in their researchers, scientists, miners, and excavation teams to the planet and all seemed to be going quite well (and lucrative!).
Then the xenomorphs, dormant for thousands of years, were alerted by the dropping of the pods and woke up. And they woke up hungry.
When Weyland-Yutani lost all contact with the entire complex, their first thought was an attack by a rival corporation, or perhaps they accidently triggered some ancient alien device, who knows? The only recourse was to hire the USMC, and send in the Marines. So they did, and with the Sulaco in orbit, surely the Marines on board could deal with whatever mishap occurred on the surface.. right?
