“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 exact origins of the Tunnelers is unknown. However, there are signs that they were created after the Great War, probably by mutation of an unknown species. Being underground predators, they build their nests deep into the earth and have lived undisturbed by surface-dwellers for centuries. Tunnelers had no direct contact with the surface world until atomic warheads beneath Hopeville detonated when the Courier unintentionally brought a package containing nuclear launch codes from Navarro. This atomic firestorm caused great earthquakes and savage storms that split open the earth, thus revealing the surface to them. Even so, Tunnelers are very averse to loud noises and bright lights. They try to keep beyond the reach of the sun by staying in tunnels and other low-light areas.
By 2281, the tunnelers are believed to pose a threat to the Mojave Wasteland in the near future, as they can spread quickly via underground tunnels, forming nests as they go. Tunnelers reproduce quickly, and hunt in packs large enough to become a significant hazard. When they are in packs, they can easily tear apart a deathclaw.
Tunnelers appear to be reptilian-humanoids with dark, scaly skin, large bio-luminescent eyes, a mouth filled with sharp reptilian teeth and chitinous spikes protruding from their shoulders and heads. Tunnelers are found underground, as their name implies, and are noted for their fear of bright lights and loud noises. Every now and then they will flee from combat and head back underground. This can be seen easily by throwing a flash bang or firing a flare gun at a tunneler. Their enlarged eyes appear to be an adaptation to the conditions of their preferred habitat and are sensitive to light; a condition which would explain their extreme aversion to weapons like flash bangs and flare guns, and to the sun itself.
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. However, 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 he or she 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. As they swarm with regular and hulking tunnelers, even a higher-level player 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 has only 25 HP less than a deathclaw, making her a formidable opponent.