Level                    Code
------------------------------
Anomalous Materials      Gordon goes to work
Unforseen consequences   Gordon teaches physics
Office complex *         Files and fear
"We've got hostiles"     Soldiers visit Black Mesa
Blast pit                Safe and sound
Power Up                 Big is beautiful
On A rail                Trains and rockets
Apprehension             Fear finds Gordon
Residue Processing       Recycle or die
Questionable Ethics      Regression brings progress
Surface Tension          Combat and death
"Forget about Freeman!"  Soldiers eat tacos
Lambda Core              Scientists at work
Xen                      Fear and gravity
Interloper               Panic in Xen
Gornarch's Lair          Xenophobia and firepower
Nihilanth                Panic and die
Endgame                  Answers beget questions