Story related and cannot be missed.
Simply complete mission 10A and it'll pop at the end of it. Mission 10A is the hardest of the bunch though. You've still got infinite respawns but there is a time limit at the end. The main objective here is to shoot the generators to open the entrance. Follow your white indicator arrow to the hole in the ground and go in. This section is difficult to navigate but here's what to do:
- Enter the hole and go through tunnel, go through either of the next two tunnels (shoot the spinning things, they'll just get in your way later), go through either of the next two tunnels (same again), go through the single tunnel (you'll see bright orange at one end).
- Follow the lava to another entrance, go through that tunnel, look for the next entrance on the other side of this area, go through that to get to the 'Core'.
- You'll get a checkpoint here. Destroy any gun emplacements and enemies, they'll just get in the way. Shoot the 6 yellow target things (the main objective). At this point I'd consider crashing the ship so that you get full health. Enter the orange cube of light (to 'drop the bomb').
- Now, you have three minutes to get back to the surface. It's not much time given that enemies will be shooting you and you've got to navigate these dark tunnels. If you can make it back the way you came, the whole place explodes and it's game over (in a good way). Get lost in the tunnels and the whole place explodes and it's game over (in a bad way).