Gordon begins the long trip across the surface areas of Black Mesa while the war plays out between the HECU and alien forces. While fighting through the aliens, and persistent ambushes set up by the HECU (due to Gordon's tracking devices in his suit), Gordon's surface journey has him swimming through a dam, navigating minefields, scale rocky cliffs, and make his way through bombed-out structures on his way to the Lambda complex.
The surface journey ends at a tactical military map that the HECU set up to aerial bomb specific targets of Black Mesa, which Gordon uses to blow up a chasing Gargantua along with blasting open a locked gate to reach the next area. It's also here that Gordon hears over the military radio that things might not actually be going well for the HECU against the alien invaders, and have resorted to just aerial bombing Black Mesa. Thus, in order to evade the aerial raids, Gordon makes his way back underground beyond the gate he just blasted open.
This Chapter contains examples of:
- Absurdly Ineffective Barricade: The HECU puts a lot of effort into intercepting Gordon, but leave exploitable weaknesses in their blockades. The minefield has obvious signs warning about the mines, the canals have a blind spot where Gordon can emerge by an IFV and establish a strong vantage point, sniper nests have obvious camo netting, and so on.
- Air-Vent Passageway: While the series as a whole is famous for it, one of the most memorable subversions happens here. You are forced to crawl through some air vents, but the Marines hear you and shoot the hell out of it. The entire vent falls off the ceiling and crashes to the ground with you inside, or if you backed up, trapped in another part of the vent with the Marines shooting the hell out of you, albeit with more cover.
- Chekhov's Gun: The tracking devices in Gordon's suit (mentioned in the chapter Apprehension), are one motive for all the traps that the HECU sets for Gordon.
- Damage-Proof Vehicle: Inverted: armored vehicles can amusingly be neutralized with even a crowbar.
- Door to Before: The section with the two security guards has a couple of these. Mainly so that you can escort a security guard to a locked supply area later on.
- Escort Mission: One of the loading maps near the end of the level requires the player to find and escort a security guard that can open up a pair of doors. The first door being the one to get out of the building the guard is found in, and the second door being the one that leads to the Gargantua chase through the garage tunnel.
- Have a Nice Death: A subtle example, but at one point you have to navigate a building packed with explosive laser mines. Should you touch a single laser tripwire, or even bump into one of the moveable boxes with a mine attached, not only will the mines explode and instantly kill you, you will then hear a bunch of secondary explosions as all the other mines explode (presumably destroying the entire building) and your vision turns all white (rather than just reloading your most recent save.)
- Hellish Copter: Every chopper the HECU deploys is optionally shot down by Gordon.
- Marathon Level: The single longest and most arduous chapter in the game. Surface Tension takes you from the mostly small, claustrophobic corridors of Black Mesa to the open desert air where shit really gets hot. It starts when you first leave the front door and throws tons of bloodthirsty marines and vengeful Xen creatures at you. Then there are a ton of helicopters ruthlessly gunning for you, a good few hard-hitting tanks looking to blow you to bits, and a segment where you have to open a flood gate while nearing drowning all while an ichthyosaur attacks you with a helicopter above water just waiting for you to surface for air. And then there's the ton of incredibly durable snipers who can rapid fire you to death, and a tripwire mine maze near the end where one errant step will spell your doom. It finally culminates in a fight with another Gargantua, requiring you to either outrun and lose it, or use discarded military equipment to order an airstrike on the thing.
- Non-Standard Game Over:
- There's a part where you have to navigate a building guarded with dozens of laser tripmines. Normally, you can usually safely destroy these tripmines by shooting them from a distance. But here, trigger even one of them and the entire building explodes; with the screen fading to white before you die.
- In addition, there's the map before the Gargantua chase that requires a security guard to open up two locked plot-forwarding doors. If the security guard dies before both these doors are unlocked, the player's screen blacks out.
- Optional Party Member: There's a map with two optional security guards on it that allow Gordon to make a small combat group for a brief time. Gordon's forced to split up from them at an alien trampoline.
- Outrun the Fireball: An encounter with a marine at the opposite end of a pipe, as you're approaching the end. There's a section where you're crawling through a narrow pipe and a soldier throws a satchel charge inside and closes the door. You have to quickly back out of the pipe before you're caught in the resulting explosion.
- The Precarious Ledge: You have to fight Marines along the side of a cliff face. It becomes extremely narrow in places and can be nerve-wracking for people with a fear of heights.
- Road Block: On the second last map of the level, there's two plot-forwarding doors that can't be opened without code access from the one security guard that Gordon finds here.
- Sidetrack Bonus: Being a massive level, there's side-areas galore throughout.
- Due to another path that leads to the wheel that opens up the sewer path to reach the cliff section, you can avoid the area of the sand Tentacle that's by the first mine field section entirely. The sand Tentacle is basically just protecting some supplies.
- Later on, there's a map section with two security guards. If you manage to keep one of these two guards alive, they will open up two locked doors to side areas for some supplies. The first supply room that has massive Suspicious Video-Game Generosity is given to the player as a security guard is right next to it. It's the second locked supply area by a stationary cannon that's a bit more difficult to get open.
- Suspicious Video-Game Generosity: Justified, there is an optional armory logically in a military barracks that is packed with enough munitions to fill all of your reserves, but be sure to keep the nearby security guard alive to unlock it. What follows is an intense series of fights with both marines and alien grunts leading up to the point where you can breach an armored door to make your way to the next area.
- Tank Goodness: HECU has brought armored vehicles to the area to take out Gordon and the Xen forces. This includes both the Bradley Infantry Fighting Vehicle and Abrams Main Battle Tank.
- Tone Shift: From claustrophobic survival against alien monsters in an abandoned series of labs-turned-charnel-house, to tense, epic open warfare against the military in the scorching desert.
- Villain of Another Story: The "Forget about Freeman" marine on the other end of a radio is met in person by Adrian Shepherd in Opposing Force. This Enemy Chatter also helps establish where Gordon is while Adrian and company are preparing to evacuate by air.
