Caverns of Mars

You control a ship descending into a narrow cavern lined with glowing emplacements and fuel silos. Using your ship's twin cannons you can destroy all of them for points, but every fuel silo destroyed also gives you 5 units of fuel. Fuel decreases over time from 100, so destroying fuel silos is necessary to make it out alive. You control your rate of descent by moving closer to the edge of the screen, but this is much more dangerous and could result in destruction. If you have extra lives remaining you respawn, but are still limited by the fuel you have. After descending far enough you are rushed by rocket ships and martian fighters, which you must destroy or dodge to avoid. After this you descend into a narrow, glowing cavern and come into contact with the reactor at the bottom. You set the reactor to detonate, and must now ascend the cavern you just entered to escape before the reactor explodes. Should you die during this sequence, you are allowed to continue, but the countdown timer is not reset.

As difficulty increases, emplacements fire back, opening and closing doors, and there are floating mines which block the cavern and must be eliminated. There are five caverns in all, each significantly more difficult than the last.