Inspired by this question, if the Hallelujah Mountains float because they're full of Unobtanium then why doesn't the company mine them instead of spending so much time and effort moving/forcing the Na'vi away from Home Tree?
The Hallelujah mountains was a very difficult location to mine. Reasons:
- Flux Vortex: The entire area was full of magnetic interference, which would cause any electronic instrumentation to mess up. There was no way to mine accurately with no electronic instruments to find where to mine.
- Terrain: The Hallelujah mountains float high above the ground, thus making it impossible to load the extracted unobtanium. Even worse, they couldn't take the usual mining trucks up there to pick up the mineral at all. There was no feasible way to collect and transport that unobtanium.
- Extraction machines: They couldn't have put their extraction machinery anywhere in that area. You couldn't use an excavator, or any standard mining equipment.
- Exploration: Since electronic navigation was not an option, most of the area was unmapped and unexplored. It would be very difficult to survey the area without electronics.
Because when they tried it, it resulted in a significant loss of life.
From the Avatar: An Activist Survival Guide factbook:
The public knows nothing about this. The RDA is destroying even these Pandoran masterpieces [The Hallelujah Mountains]. A score of personnel were killed during unobtanium extraction, when a ship-sized formation above them suddenly inverted and crashed down on the workers. Removal of unobtanium had likely disrupted magnetic fields around the mountain, but some maintain it was an act of "balancing" initiated by Eywa
The mountains where not extra dense with Unobtanium, what was extra dense was the strength of the magnetic field around the mountains and that is what caused the Unobtanium in the rock to float.
This point is brought up when the instruments stop working when they first fly in to the mountains.
From the movie transcript:
THE SAMSON is dwarfed by enormous ARCHES OF ROCK. GRACE See these magnetic formations. We're getting close. TRUDY Yeah we are. Look at my instruments. On the dash, many of the displays are fritzing out. 58. GRACE Yup. We're in the flux vortex. AHEAD, a cloud bank parts, revealing -- THE HALLELUJAH MOUNTAINS. Right in front of them. NORM Oh. My. God. Jake leans forward between the seatbacks for a good look out the front canopy. JAKE'S POV -- enormous islands of rock are hovering a half mile above the ground. They are overgrown with rainforest, and straggly beards of vines hang down beneath them. Waterfalls stream down the sides and dissolve into spray at the bottom. ON JAKE, staring in amazement. It is both awe-inspiring and disturbing. Trudy turns, grinning at Jake. TRUDY You should see your face. WIDE AERIAL -- the Samson is tiny as it approaches the floating islands of rock. An archipelago among the clouds, they cast great shadows over the forested slopes below. JAKE (V.O.) Yeah, so what does hold them up? Grace explained it to me -- some kind of maglev effect because unobtanium is a superconductor, or something. At least somebody understands it. Just not me.