If you come across a level you don't want to play then during gameplay press the '~' (tilde) key to bring up the console and then enter the following passwords without the quotes to skip that level in the game.

Unlock Ammo Depot Level:
Type 'Map rammo2'.

Unlock Lower Mines Level:
Type 'Map rmine1'.

Unlock Munitions Plant Level:
Type 'Map rammo1'.

Unlock Research Hangar Level:
Type 'Map rhangar1'.

Unlock Tactical Command Level:
Type 'Map rbase2'.

Unlock Mine Engineering Level:
Type 'Map rmine2'.

Unlock Tectonic Stabilizer Level:
Type 'Map rlava2'.

Unlock Maintenance Hangars Level:
Type 'Map rhangar2'.

Unlock Eastern Warehouse Level:
Type 'Map rware1'.

Unlock Logistics Complex Level:
Type 'Map rbase1'.

Unlock Thaelite Mines Level:
Type 'Map rlava1'.

Unlock Waste Disposal Level:
Type 'Map rsewer2'.

Unlock Waste Processing Level:
Type 'Map rsewer1'.

Unlock Widow's Lair Level:
Type 'Map rboss'.

Unlock Waterfront Storage Level:
Type 'Map rware2'.