Pleiades Iron-ore mine is probably the smallest of all of these facilities. It is relatively devoid of secret areas, but the ones that you can find are quite useful. In this level, you will need to work fast to survive, although if you clear as much of the mine as possible before you destroy the reactor it is significantly easier. This is the only level from which you can reach the yellow-access area of Lapis Lazuli.
You need to obtain the yellow key, blow the reactor and then destroy all of the mini-reactors and the triggers behind them, and also pick up the blue key, before you can acquire the red key and exit the mine.
To make it easier to get the red key in time, you can blow the mini-reactor below the exit and clear out the rest of the accessible mine. Keep in mind that you should go straight for the secret level once you have the yellow key - you cannot reach it any later.