- "The Hollow is a vast subterranean world beneath the surface of Sera, filling the surface with a maze of underground caverns and caves. Once considered the realm of mysterious and terrifying beasts, the Hollow has now been extensively explored."
- —Settlement 2's Natural History Museum Exhibit on the Hollow.
The Hollow is an underground tunnel network beneath the surface of Sera that was home to indigenous, subterranean creatures as well as the Locust Horde. Located in the crust of Sera, the Hollow is separated between the Inner Hollow and Outer Hollow.
The Outer Hollow is closer to the surface and was explored by humans after the invention of the Lightmass Process to refine Imulsion for fuel. The Inner Hollow lays deeper underground, roughly 3 kilometres beneath the surface, and was the cradle of the Locust civilization. The current fate of the Hollow is uncertain, after the Lightmass Offensive destroyed much of the Outer Hollow while the Inner Hollow was flooded when Jacinto City was sunk underground and filled with seawater.
The Hollow is the name for the underground network of tunnels that lay beneath the surface of Sera. It has been the Serans dumping grounds for hundreds of years. The Hollow was the home of the Locust until they breached the surface of Sera on Emergence Day. Massive and complex, the Hollow and its tunnel network have yet to be completely mapped.
Aside from natural voids and caves in the planet's crust, the Hollow network in Sera was created by gargantuan creatures known as Riftworms. These 10-mile long creatures burrowed beneath the surface - constructing the large tunnel system seen in the Hollow. Because of the excrement left by the Riftworms, it allowed for life to be created in the Hollow. A large and complex eco-system was established in the Hollow. At some point, the Riftworms went extinct or dormant for years. Meanwhile, smaller creatures such Rockworms, Corpsers, and Seeders were responsible for further tunneling and creating the Hollow network.
Humanity was aware of the existence of at least the Outer Hollow for a considerable length of time before Emergence Day, as they used it as dumping grounds. Several areas of the Outer Hollow were explored and mined for resources, such as petroleum or osmium. During the energy crisis on Sera, it was discovered that Imulsion, a low-viscous, naturally occurring liquid fungus could be refined into usable fuel through the Lightmass Process. After the creation of the Lightmass Process and the start of the Gold Rush, Serans began mining the Outer Hollows for Imulsion. During the Pendulum Wars, it was a restricted area. However, it also triggered the Pendulum Wars, in which nations without natural Imulsion reserves, the Union of Independent Republics, lost economic and political power to nations that had majority of reserves, the Coalition of Ordered Governments, and fought for ownership of Imulsion.
During the Pendulum Wars, miners of Imulsion and their families began to contract a fatal disease known as Rustlung, caused by overexposure to Imulsion fumes. Fearing the economic and political ramifications of using a toxic fuel source, the Coalition transported the children of Imulsion-poisoned miners to the New Hope Research Facility, where they could be studied and treated. Under the supervision of geneticist Dr. Niles Samson, he attempted to not only treat the children but evolve humanity into superior beings by mutating them with the genetics of the indigenous creatures of the Hollow, resulting in mutated humans known as the Sires. Due to the unethical experimentation, employees leaked information about the facility, resulting in the Coalition shutting down the facility and indicting all involved. However, a Fringe COG Party built a secret lab in the Hollow under Mount Kadar where Niles Samson could transfer and continue his work.
Continuing his work underground, Niles Samson spliced the DNA of the Sires with the embryonic stem-cells of Myrrah, a child of an Imulsion miner born with an immunity to Rustlung. The splicing resulted in the birth of a new race of hybrids, the Locust Horde. Because Myrrah's stem-cells were used in the creation of the Locust, she was able to telepathically communicate and command them. She later gave birth to a human daughter named Reyna, but when the father escaped the facility with Reyna, Niles Samson lied to Myrrah that Reyna had died during the escape. Her grief and anger resulted in her commanding the Locust to break-out and rebel, massacring the human scientists and abandoning the facility. Myrrah, taking upon herself to become the Locust's Queen, led the Locust Horde to independence - taking them deeper into the Hollow to avoid human persecution and to start their own civilization.
Queen Myrrah led the Locust deeper into the Inner Hollow beneath Mount Kadar and began to construct their own civilization: Nexus. Using discarded materials and technology from human dumpsites in the Hollow, the Locust managed to construct the Royal Palace with surrounding cities, temples, and fortresses. They also managed to recycle materials to construct their own technology to build a society with its own language, religion, culture, military, and government. To build their army, the Locust began to capture and enslave natural creatures of the Hollow, genetically engineering them to act as weapons and transport. In 9 B.E., biologist, Doctor Elain Fenix, began to investigate the Inner Hollow following the discovery of mutated Rockshrews in 17 B.E. She then discovered the Locust Horde, but before she was able to reveal her findings to the public, she was executed by the Locust.
In 7 B.E., the effects of Rustlung returned to affect the Locust Horde. The exposure to Imulsion overtime led the Locust to become hosts for the Imulsion parasite, becoming Lambent. Aggressive and mutative, the Locust infected with Lambency were compelled to infect or kill those not infected. This resulted in the Lambent War, with the Locust Horde forced to fight their infected brethren. Because the Locust either suffered losses or became infected, the Locust quickly began to lose the war. Following her notes, physicist Professor Adam Fenix, husband of the late Elain Fenix, also discovered the Locust Horde. However, he was spared when Queen Myrrah believed he could help solve the Lambent crisis without harming the Locust. During this time, Adam Fenix spent extensive time in the Inner and Outer Hollow, plating Geobots to map the Hollow, and studying the effects of Imulsion.
However, Adam Fenix was unable to solve the Lambent crisis due being conscripted to build weapons of mass destruction for the Coalition of Ordered Governments Army and due to it being impossible to eradicate the Lambent without harming the Locust Horde and their ancestry linked to Imulsion-poisoned humans. Queen Myrrah then sought to invade the surface, eradicate the humans, and colonize Sera to abandon the Lambent. The Locust Horde then emerged on the surface on Emergence Day, waging a 17-year war against humanity known as the Locust War. Due to the Locust being underground, most of the Outer Hollow was abandon by Serans to avoid the enemy. The Locust continued to occupy and tunnel underneath the surface of Sera to emerge and slaughter all humans. The only place the Locust could not dig through, other than islands in the oceans of Sera, was the Jacinto Plateau. Consisting of granite bedrock, cities on the plateau such as Ephyra and Jacinto City were secure from the Locust.
For nearly ten years, the Locust attempted to attack the last safe cities by digging through natural fissures in the bedrock. In 10 A.E., the Locust were successful in emerging and occupying the Coalition capital city of Ephyra, forcing humanity to relocate to Jacinto City. Due to advice given by Adam Fenix, it was believed that by sinking Jacinto City, the surrounding seawater would flood the sinkhole and thus flood the entire Inner Hollow, killing all life below the surface. Queen Myrrah then set her sights on sinking cities on the Jacinto Plateau in order to weaken the bedrock and give the ability to sink Jacinto City. Meanwhile, the Coalition constructed the Sonic Resonator and the Lightmass Bomb in Gale, 14 A.E. In the Lightmass Offensive, the Coalition attempted to use the Sonic Resonator to map the Locust tunnels in the Outer Hollow and then use the Lightmass Bomb to destroy the heart of the Hollow. The Sonic Resonator failed to map the complete Outer Hollow, but the data-mapping provided by Adam Fenix's Geobots allowed the Coalition to properly destroy the Outer Hollow.
The Lightmass Offensive successfully destroyed the Outer Hollow, but failed to destroy the Locust stronghold. Additionally, the Lightmass Bomb awoke a dormant Riftworm, allowing the Locust Horde the ability to sink entire cities and weaken Jacinto's base. The Riftworm was used to sink the cities of Tollen, Montevado, and Ilima on the Jacinto Plateau, allowing the Locust to advance and attempt to sink Jacinto City. In response, the Coalition began Operation: Hollow Storm, a campaign that sent a majority of Coalition troops to the Inner Hollow and personally destroy the Locust stronghold, but initially failed due to insufficient intel. Obtaining declassified intel from the New Hope Research Facility, the Coalition discovered that the Locust were located beneath Mount Kadar, allowing the Coalition to attack Nexus. When the Coalition found Adam Fenix's research, they decided to sink Jacinto for themselves before the Locust and Lambent could escape the Hollow. Using a Lambent Brumak, the Coalition was successful in sinking Jacinto City and flooding the Hollow. A majority of the Locust Horde and indigenous creatures of the Hollow were killed during the flooding.
In the aftermath of these events, it is unknown if any parts of the Hollow remain intact. Any Locust that survived fled the Hollow and turned to living on the surface as the Savage Locust while others became the Queen's Guard. The Imulsion from the Hollow was raised to the surface by the flood, resulting in the Lambent Pandemic. However, the Imulsion Countermeasure Weapon created by Adam Fenix eradicated all Imulsion and Lambent lifeforms, and neutralizing the Locust Horde for twenty-five years.
During this time, it is unknown about the current state of the Outer and Inner Hollow. However, some underground sites in the Outer Hollow remained intact, such as Osmium Mine Bravo. The Hollow and its creatures were a focus of study in Settlement 2's Natural History Museum.
- "The surface of Sera is honeycombed with a dense network of caves, tunnels, aquatics and caverns."
- —Settlement 2's Natural History Museum Exhibit on the Outer Hollow.
The Outer Hollow is the outermost part of the Hollow near the surface. Barren of fauna and natural life other than Imulsion, it has been largely explored by human miners and had miles of piping and mining stations built across the subterranean landscape. It was a known fact that miners had shorter lifespans than the average human due to the hazards of Imulsion mining. This part of the Hollow was also the dumping ground of the human race. Ironically, because of this the Locust Horde discovered and developed many of their weapons which allowed them to attack the humans years later.
Water flows in rivers throughout the Hollow, and every surface is constructed of rocky terrain. Stalactites and stalagmites are common, sometimes in such size to create full columns. The Outer Hollow was occupied by the Locust Horde shortly before Emergence Day, but was destroyed by the Lightmass Bomb during the Lightmass Offensive in Gale, 14 A.E. Some portions of the Outer Hollow remained intact by 42 A.E., such as the osmium mine in Fort Reval and the Orzabal Crater, which were turned into Locust burial sites following the end of the Locust War and subsequently became one of the first Swarm Hives.
- "Below the Outer Hollow, the cave systems join to form huge caverns, often flooded with enormous underground lakes."
- —Settlement 2's Natural History Museum Exhibit on the Inner Hollow.
The Inner Hollow is the deepest parts of the Hollow, estimated to be three kilometers below the surface. Unlike the Outer Hollow, the Inner Hollow has natural flora and underground lakes, and was inhabited by indigenous creatures and large Imulsion reserves. Human exploration hardly reached the Inner Hollow except for the scientists of the Mount Kadar Laboratory. Following a violent uprising against their creators, the Locust Horde colonized the Inner Hollow and created a vast civilization known as Nexus.
However, the Inner Hollow began to become overrun with an epidemic of Lambency, in which the Imulsion was able to infect, control, and mutate hosts into infecting and killing other lifeforms. The Inner Hollow was invaded by the Coalition of Ordered Governments in 14 A.E. and was flooded entirely with seawater after the city of Jacinto was sunk underground by the COG to drown both the Locust Horde and Lambent lifeforms.
- Main article: Hollow Creatures
It is unknown if any other creatures in the Hollow existed before or during the Riftworms and Rockworms, but the burrowing of these massive worms allowed for life to grow in the Hollow due to fertile land created from the manure of the worms and the naturally occurring lakes in the Hollow. Many creatures were able to evolve in the Hollow into different species of animals. Some species in the Hollow had been discovered, but most resided in the Inner Hollow and had not yet been explored by humans. Occasionally, creatures from the Hollow would emerge to hunt as ancient Tyran tales tell of a six-legged creature that burrowed underground and would emerge to hunt and devour naughty children. Some of the creatures were genetically-engineered by the Locust scientist Ukkon as weapons for their war effort such as Corpsers and Brumaks.
- Gears of War: Anvil Gate