“Since light travels faster than sound, people appear bright until you hear them speak.”
Unknown
“Now, now my good man, this is no time for making enemies.”
Voltaire on his deathbed when asked by a priest if he renounced Satan
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Vents are an integral aspect of NS mapping for their role as alien-only routes. Since nearly all the alien classes can either walk on walls or fly in some fashion, aliens can make use of any route regardless of its location. Marines, on the other hand, are grounded like any other FPS human character, unless they have the late-game Jetpack technology.
The ventilation system of Nexus was planned from the very beginning. The image to the left shows the original plan for the layout compared to the newest released version's minimap. Take note of the significant changes in the map layout, discussed in Dev History, which caused some parts to move or even be completely remodelled.
I wanted almost the entire map to be accessible from the main vent system, much like the map Ns_Nancy. While Ns_Nancy's vent system has amazing scope, it suffers horribly from overcomplexity. It's lack of detail also creates a disorienting effect. As such, I planned to avoid the pitfalls found in Nancy's vent system through centralization and detail.
To counter overcomplexity, Nexus' vent system is centralized. The Infested Hub is the hub of the whole system. With five exits that branch off to their respective corners of the map, the players only need to memorize this one small room to be able to navigate their way about the vents.
When used in conjunction with the minimap, a player can readily see the entire layout of the vents as they are coloured red instead of the standard blue. To avoid confusion with the main layout, small parts of the vents are hidden under the normal paths.
The Second Hub is just what its name says; a second small hub. Only connecting three branches, one of which directly goes to the Infested Hub, it is an even smaller room, but can still fit one structure. As such, there are two areas in the vent system where the marines can build a phasegate: the Infested Hub and the Second Hub. (the small area mentioned in the Hidden Routes section, further down the page, is part of a much smaller, separate vent system)
While most maps simply use one texture for the entirety of their all-square vents, I tried to extend my attention to detail, already found throughout the map, into the vent system as well. Many areas of the vents are still simply square tight halls, but I tried
to keep them interesting and cohesive by employing more realistic and 'sexy' textures (such as heavy wiring and tech-plating) in conjunction with different lighting schemes and varying elevation levels. Some parts require a marine to crawl, and a few sparse areas were blacked out for a terriying effect. The Infested Hub (and to a small extent, the Secondary Hub) itself is also a novel concept for a vent system, as it is a small gunk-ridden room hidden away in the bowels of the ship.
Rather than simply feeling like 'random vents that the mapper placed for gameplay use', these vents truly create an experience of running around behind the access panels of a real spacecraft.
Vents may be meant for aliens, but they double as ninja routes for the marines. The term Ninja has a very specific definition in Natural Selection. A ninja is a marine who manages to sneak deep into alien territory and build a phasegate near a hive location, allowing the entire marine team to phase over and rush the hive.
With such an expansive vent system, the potential for Ninjas is massive. I attempted to not only encourage such use of the vents, but to also balance their power. Marine access within the vents is rarely hampered; the majority of the vent system is tight, but high enough for a marine to run through. But for a marine to actually get into the vents is difficult.
Near Marine Start, the only access is via the Coordinate Mapping jump puzzle (explained in the next section. As a very difficult puzzle requiring skill, teamwork and even a small amount of resources, this is a well balanced yet rewarding access point.
The other access points are spread out, and would really require live demos to explain. An example is found in Port Shaft, where a player may jump from a moving elevator to a small opening, risking a far fall if it is missed. Each of the hive locations have access points as well, though some of these require their own small ninja routes to be followed. Again, I've attempted to explain these in the next section.
A mapper can place hidden routes by simply blending a jumpable ledge in with its background. That ledge could lead to another, which would eventually lead to a vent, etc. Being on an engine which offers peculiar air movement abilities, 
a mapper could even take the concept further by employing more difficult jumps requiring such techniques. Or perhaps a certain route could simply require a boost by another marine. I made use of all types.
Some routes simply offer better vantage points. The semi-hidden ladders behind the far pillars in Chromo allow any grounded players to reach to far upper levels of the room, whereas a very sneaky route in Port Thruster allows a marine to get far above the hive.
There's a particular route from the top floor in Residential Block where a marine only needs a boost to access a small vent system near Port Thruster hive. 
Within this vent system is a very tiny room, which just happens to be perfectly big enough to fit a phasegate. Once built, the entire marine team can phase in and storm the hive... after first crawling through 30 feet of cramped vents. As I led a 15 man team through this assault one time in a real game, every single one of them screamed in excitement about the 'splinter cell s***' we had just pulled off.
The secret route in Engineering requires no special jumping or trickery; it simply requires map knowledge, patience, and some courage. Without the explanation given here, it would take a regular player quite some time to even realize this existed. The end result would likely be the driving force for a player to discover this route; by completing this jump puzzle, a player gains access to the Manual Override [explained in the Scripted Events section], opening two new siege locations.
The route in Railgun Weapons Systems is rather minor, but can create some rather amusing situations. A marine can gain access to a small platform hanging from the ceiling, allowing him to defend, ambush, or even build. There is just enough room for a phasegate up there (though marines would have a helluva time trying to phase out!).
The south-west siege room Docking Wing holds one of the tougher jump puzzles in the map. The first jump can be done solo with difficulty, or can be bypassed with a boost; the player must then perform a series of other jumps, and even cross an area where he would be extremely vulnerable. In the end, he is granted access to the small isolated vent system that the aliens use to counter the siege on Port hive, also allowing him to enter the hive room at the very back.
As mentioned in the Vent System section above, Marine Access to the ventilation system needs to be controlled, yet still available. Chromo Hive is the only location where marines can access the vents with relative ease. There are five other locations throughout the map where the marines can gain access with moderate difficulty. As shown in the image to the right: a perilous jump from a moving elevator in Port Shaft; a boost on the second floor in Double; an interesting route can be taken in either Thruster Hive; or a jump to a small well-hidden vent in Keystone Interface. None of these require extreme techniques.
We now arrive at the Coordinate Mapping access point. Situated directly outside of Marine Start (overview), this access point can be a very powerful tool for the marine team. As such, it had to be balanced with an equally difficult jump puzzle. It first requires a boost and a welder to 'open' permanently. Once open, a player would have to successfully complete seven total jumps from start to finish. The second last is rather difficult, and the last is downright sadistic, requiring a 'strafe jump' to complete, which is a skill-based exploit of the HL1 physics engine. Since I am considerably skilled at executing such a jump, I tinkered with this jump distance until I could only successfully complete it roughly 1/3rd of the time. The vast majority of players can never complete it. But they can beg for jetpacks.
There is one final hidden access point for the ventilation system, and it is found in Slug Room. It is of extreme difficulty, and I have decided to not go into its details in this explanation. See if you can find it :)
Next Page: Engine Limits, Dev History,
Future Releases and the Final Word
