“They'll start emerging throughout the Mojave in time, might be years. Probably less. They breed fast, hunt in groups, more than enough to bring down the strongest in the Mojave. Once they draw blood... Seen them tear apart deathclaws... Deathclaw might get some, but the rest will swarm it, tear it apart, like Denver hounds.”— Ulysses
The true origins of tunnelers are not known. Ulysses speculates that they may be a result of the Great War.  After the bombs fell, radiation heavily mutated them, adapting them for an underground life. These mutations resulted in a form of bioluminescence. They evolved into quadrupedal predators that lived and bred beneath the sands of the Divide. There they remained, completely unaware of the surface world for nearly two centuries.
When the Courier was contracted by the New California Republic to bring a package from Navarro, they unintentionally caused the detonation of many of the nuclear warheads that had never been launched, slumbering in their silos since the Great War. The subsequent explosions and earthquakes split open the ground, releasing tunnelers into the world above. However, due to their adaptations for life underground, they are hypersensitive to bright lights and loud noises. This leaves them unable to venture outside for extended periods of time, so they remain in the tunnels they have dug or other areas free of sunlight.
By 2281, according to Ulysses, the tunnelers may pose a significant threat to the Mojave Wasteland in the near future, due to their ever expanding network of underground tunnels and nests. They reproduce quickly and hunt in large packs, enough to pose a significant hazard to the toughest of opponents, even deathclaws.
Tunnelers appear to be reptilian-humanoids with dark, scaly skin, large bioluminescent eyes, a mouth filled with sharp teeth, and chitinous spikes protruding from their shoulders and heads. Tunnelers live underground, and are averse to intense sensory stimulation. This can be taken advantage of through the use of flash bangs or firing a flare gun, causing them to flee from combat and head back underground. Their enlarged eyes appear to be an adaptation to the conditions of their habitat. The tunneler's bite is known to be as deadly as a bark scorpion sting.
Tunnelers will flee from a multitude of things such as:
Flash bangs: These will startle them and cause them to run away or become frenzied. They must detonate at a close proximity to have any effect.
Flare gun: Tunnelers will only be frightened by the flare gun if they are hit. When hit they will flee as long as they are on fire; if the flare misses, and the shot lands at their feet, they are less likely to do so.
Flamer: Although not directly damaging them with any effects other than fire damage, firing a flamer at them will usually cause them to burrow and resurface in another location.
These are the most common type of tunneler and will deal medium damage individually. Tunnelers typically attack in groups, swarming anything in their way. This makes them lethal opponents to even higher-level Couriers.
Hulking tunnelers are the second largest variant of tunneler, second only to the tunneler queen. They are quite rare and are not as fast as regular tunnelers, but if they strike the Courier, there is a good chance they will be staggered or knocked down.
The smallest of the tunnelers, they are also quite rare except at higher levels, when they are more frequent in the pack. Venomous tunnelers are faster than others and their attacks deal less damage on the initial strike. As their name implies, they possess a deadly venom which can quickly overpower an inexperienced player character. As they swarm with regular and hulking tunnelers, even a higher-level player character can be killed in a matter of seconds.
The largest of the tunnelers, the queen combines deadly speed and hard-hitting attacks, enough to kill even a deathclaw. While there is only one (located in the Cave of the Abaddon), she only has 25 less HP than a deathclaw.
↑The Courier: "Radiation from the War might have created them. Then detonations, quakes, caused them to surface?" Ulysses: "[SUCCEEDED] Makes sense. Truth favors that, matches the history of this place.Were signs they were here for a long time, before Marked Men, before what happened to the Divide cracked their sky. If so, they were buried deep. Might have been born the day America's embers started to settle. Seen worse things twisted by radiation." (Ulysses' dialogue)
↑The Courier: "What were the creatures in the underpass?" Ulysses: "Tunnelers. Predators that make their own roads beneath the ground here. Divide broke their sky, showed them the world above - and the scent of new prey. Be a slower death for the Mojave than bombs and fire... but they'll come for its people, from where they least expect - below." (Ulysses' dialogue)
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↑The Courier: "Any more recipes.. or histories... you can teach?" Ulysses: "Antivenom's helpful around the Mojave with the Bark Scorpions. Even one sting can kill you. Tunneler bite can be just as bad. My tribe, Twisted Hairs, had a better way than Antivenom, if you act fast. Snakebite Tourniquet - cuts off the venom, no need to scavenge glands and blood. Or drink that mess. Part of the reason we wore our hair like we did, like I do now. Here's some for the road - trick to making it's not hard. Get bit, hook and twist it. Fast as blinking, doesn't give poison time to sink in." (Ulysses' dialogue)