A notable area of the tunnels was a connection point where various pathways intersected. Mike and Steve referred to the area as a "hub". According to the then possessed Will, the Mind Flayer considered the hub to be "important".
As Chief Hopper remarked, the hub appeared to be a "graveyard" of sorts; dead and decomposing bodies of animals were strewn across the floor. The hub also appeared to contain piles of shedded Demogorgon skin.
The Mind Flayer had control over a strange, flower-like orifice embedded into one of the tunnel walls. He used this orifice to disperse spores into the air; these spores could be released either in the form of mist or as highly concentrated pockets of gas.
A team of Rift soldiers headed to the hub, only to find themselves blinded by mist released by the spore clusters, before being violently savaged by the Demodogs. On two other occasions, both Hopper and Dustin fell victim to the toxic spores released by the spore clusters. It seems likely that, via his control of the spore clusters, the Mind Flayer played a direct role in shaping these events.
- Main article: Vines
One of the biological outgrowths of the Upside Down was a series of sensitive vines that concealed the entrance to the tunnels from the the outskirts of Hawkins.
When Jim Hopper was attempting to dig out a way from the tunnels, the vines prevented him from doing so and instead entangled him up. These vines also displayed extreme vulnerability to fire as seen when ignited by the soldiers or the kids.
In November 1983, an inter-dimensional gateway was opened at Hawkins National Laboratory. As a result, an alien biological growth originating from the mysterious Upside Down began to spread through and consume the lower levels of the lab.
By October 1984, a series of interconnected tunnels had formed as a result of the Upside Down's intrusion. The tunnels stemmed from the Gate before branching out and spreading under the outskirts of Hawkins.
After being possessed by the Mind Flayer, Will began experiencing what he called “now-memories”, where he could see and know what the Mind Flayer could. Will began drawing these visions in order to better understand them. At first, they appeared to be random scribbles, but after being assembled in the correct order, they depicted a strange maze-like pattern. Believing the drawing depicted vines, Hopper left, heading to the pumpkin patch.
After digging into the pumpkin patch's soil, Hopper descended into a subterranean tunnel overgrown with biological matter originating from the Upside Down. As he was exploring, he encountered a strange growth that sprayed some type of spore-spray in his face, causing him to lose consciousness.
The next morning, Will informed Joyce that Hopper was in trouble after seeing him through his connection with the Mind Flayer. While they were able to figure out where Hopper was in Will's drawings, they did not know the actual location. Just then, Bob arrived, and because of his talent in solving puzzles, Joyce enlisted his help. Bob realized that the drawings were a map of Hawkins, and was able to pinpoint Hopper's location.
When Hopper awoke, he vomited a dark substance. Realizing he was trapped, Hopper continued through the tunnels looking for a way out. He eventually came to a point where several of the tunnels intersected, filled with several animal carcasses. Discovering that the vines were vulnerable to fire, he fashioned a makeshift torch out of a piece of an animal carcass and one of his sleeves. As he futilely tried to dig a way out, he was overcome by exhaustion and stopped to rest. He was then ensnared in a mass of vines. Eventually, he was found by Joyce and Bob. After Hopper was freed, soldiers from the lab arrived and urged them to leave shortly before burning the tunnels. Suddenly, Will, who was waiting above ground with Mike, collapsed and began convulsing on the ground.
Will was taken to Hawkins Lab were it was discovered he had some sort of “virus” that connected him to all other infected hosts in a hive intelligence. Back in the infirmary, Will claimed he knew of a way they could possibly stop the shadow monster. Using images that the lab personnel had taken of the map, Will directed the scientists to a large area in the tunnel system, stating “he doesn't want me to see there.”
As soldiers traversed the tunnels, Will revealed that the Mind Flayer made him deceive the soldiers. Realizing it was a trap, Mike tried to warn the scientists, but it was too late; the soldiers were ambushed and killed by a pack of Demodogs.
In order to provide a distraction so Eleven and Hopper could safely reach the Gate, Mike, Lucas, Dustin, Max, and Steve went into the tunnels to burn them. Making their way to the hub, they doused the entire area in gasoline and set it ablaze. On their way out of the tunnels, the group encountered Dart. Recognizing Dustin, Dart did not attack and was easily distracted by Dustin giving it another Three Musketeers bar, allowing the group to escape.
When the Gate was closed, the tunnels, as well as all organic matter originating from the Upside Down inside them, died.
After the Mind Flayer latched onto Will Byers, Will struggled to put his feelings and thoughts into words as the entity slowly took possession of him. Joyce encouraged Will to draw out his thoughts rather than speaking. Will then created a series of drawings, that when assembled in the correct way, revealed a map of the underground tunnels. Bob Newby was the first to realize the nature of the drawings, figuring out that many of the negative spaces between the lines represented bodies of water. Bob identified the following water-based locations in Hawkins:
- Lovers' Lake
- Lake Jordan
- Sattler Quarry
- The Eno River
The map was originally assembled somewhat haphazardly across the walls and floor of the Byers house. Hawkins Lab later photographed the map and made a composite image.
Behind the scenesProduction designer Chris Trujillo along with the members of the art department decided to craft the series of tunnels on a sound-stage without digitally creating them via green-screen, as it was important that the actors had real physical spaces. They spent a long time to build approximately 80,000 square feet of tunnels. The tunnels were constructed using a series of six to fourteen feet-diameter wooden ring sets. But since there was nowhere to place a light so far underground, cinematographer Tim Ives decided to use a back light made of plywood rings by wrapping with chicken wire that would encompass the tunnels. They used three pounds of foam throughout the entire interior surface to give it a more organic structure, while keeping it scary. 
The VFX team had to design effects for all the flora of the tunnels with third party vendors and had to layer them over the filmed scenes so as to make the final result look like an eerie graveyard.
- ↑ 1.0 1.1 1.2 In an interview with TV Guide Paul Graff, a member of the VFX team described the tunnels as such: "The tunnels are in the real world but they're basically an organism by itself. They aren't made out of earth, they have a skinny exterior because it's literally like a creature itself."
- ↑ "‘Stranger Things’: Hawkins Gets More Colorful and the Upside Down Grows Darker in Season Two" Indie Wire. July 25,2018 .
- ↑ ""
|The Upside Down|
|Locations||The Upside Down • The Gate • Hawkins • Hawkins National Laboratory • Public Library • Portals • Hawkins Middle School • Hawkins tunnel system|
|Entities||D'Artagnan • The Demogorgon • Demogorgon (species) • The Mind Flayer • Slug-like creature • Hawkins tunnel system • Vines|
|Visitors||Joyce Byers • Will Byers • Eleven • Steve Harrington • Dustin Henderson • Barbara Holland • Jim Hopper • Max Mayfield • Bob Newby • Shepard • Lucas Sinclair • Mike Wheeler • Nancy Wheeler|
|Miscellaneous||Upside Down egg • The Upside Down (episode) • Dungeons & Dragons • The Void|
|Articles may contain spoilers.|