- "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 was an underground tunnel network beneath the surface of Sera that is home to the Locust Horde and many other subterranean creatures. Located in the crust of Sera, the Hollow is split between the Inner Hollow and Outer Hollow. The Outer Hollow is closer to the surface and were explored by humans after the invention of the Lightmass Process to mine for Imulsion. The Inner Hollow lays deeper underground and appeared to be 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 sank. Both areas of the Hollow contained subterranean creatures.
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 is the home of the Locust and was their home 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.
Humanity was aware of the existence of at least the Outer Hollows for a considerable length of time before E-Day, as they used it as a dumping ground. After the creation of the Lightmass Process and the start of the Gold Rush, Humans began mining the Outer Hollows for Imulsion and presumably for other useful materials. During the Pendulum Wars, it was a restricted area.
The Lightmass Process soon triggered a global war on the surface that waged for decades, with both sides likely relying on resources taken from the Outer Hollows to fuel their war efforts. Meanwhile, in the Inner Hollows another war was being fought on a similar scale between the Locust Horde and the Lambent Locust. It is likely that while the Human forces did not encounter any Locust presence throughout this period, the Locust did notice the Human activity. The devastation caused to the environment by the Humans is a possible reason as to why Queen Myrrah decided to wipe out the Human race in an effort to win the war in the Inner Hollows.
As the Pendulum Wars came to a close, the Locust Civil War continued, with the Lambent Locust gaining the upper hand. Shortly after, the Locust Queen ordered the invasion of Sera's surface, triggering the Locust War which would eventually lead to the devastation of the Hollow.
After E-Day, the Humans continued to mine Imulsion from the Hollows to fuel their fight for survival, but the process was now done on a much smaller scale. The Lambent Locust also continued to advance against the Horde, likely conquering large parts of the Inner Hollow. Eventually, 14 years after the Locust's attack, a Human offensive was launched which resulted in the destruction of the Outer Hollows. Several months later in the year 15 A.E., as the Lambent assaulted the Locust capital of Nexus, the COG sank Jacinto City which caused the Inner Hollows to flood with seawater. 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 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.
This part of the Hollow has been largely explored by human miners and had miles of piping built across the subterranean landscape. It is a known fact that miners had shorter life spans than the average human, maybe from the hazards of Imulsion mining or Hollow creatures. 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. The appearance of The Hollow seems to be almost uninhabitable by human standards. Kryll excrement litters the ground, meaning that the area has a foul stench. It can also be surmised that, as Imulsion is very hot, the heat must only amplify the smell. Water flows in rivers throughout The Hollow, and every surface is constructed of rocky terrain, which has almost a viscous texture to it. Stalactites and stalagmites are common, sometimes growing in such size to create full columns between the floor and the ceiling. There is a red, fluorescent fruit that grows on the walls and ceilings of The Hollow, which the indigenous creature known as the Rock worm feasts on. Much seismic activity, often caused by the Riftworm, has devastated the landscape of The Hollow, causing major rifts in the ground, and constant collapse of the environment. It is later revealed that The Hollow is so massive that the COG resonator is only able to map a fraction of the Locust tunnels. Steam pours from numerous rifts in the tunnels, illustrating either that the entire environment is extremely hot, or that that the ground beneath the The Hollow has a naturally high temperature. The Locust Horde transport to the surface of Sera through the use of Emergence Holes, which are spread throughout the surface of the planet, ranging in size anywhere from 8–13 ft and a depth of 6–20 ft.
- "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. Unlike the Outer Hollows, the Inner Hollows have natural fauna and flora with indigenous creatures ranging from Rockworms to small beetles. It is assumed the Locust originated from the Inner Hollows as their capital, Nexus, is located here. The Riftworm also lay in hibernation in the Inner Hollows until it was awakened by the Lightmass Bomb.
The Inner Hollows contain natural lakes with various fish including the Leviathan, and Imulsion lakes. It seems human exploration never reached this far into Sera's crust except for a few Stranded groups who migrated down there to escape the war between the two factions. Unknown to the COG, another Locust faction, the Lambent, shared the Inner Hollow with the Locust Horde and were in the midst of a brutal civil war. During Operation: Hollow Storm, thousands of Coalition troops were dropped into the Hollow and were cut off from the surface, leaving hundreds dead or captured. This led to the Locust counterattack at Jacinto, and its eventual sinking which caused the flooding of the Inner Hollows.
Creatures native to the Hollows include the Locust and the menagerie of creatures they use in their army. However, there are multiple other creatures that aren't part of the Locust Horde that also inhabit the Hollows. In the aftermath of the destruction of the Outer Hollows and the flooding of the Inner Hollows via Jacinto's sinking, the fate of many indigenous creatures is unclear. Creatures utilized by the Locust Horde escaped amongst the Savage Locust, whilst aquatic creatures such as the Mangler and Leviathan have been sighted in Sera's Oceans. Any creatures that could not escape the Hollow may have been wiped out, although it is not known.
The Locust use these beasts to steer the Locust Gunboat. They appear to be native to the Inner Hollow, but since the sinking of Jacinto they have been sighted in Sera's oceans.
These semi-giant worms inhabit small caves within the Inner Hollows. They are protected from most hazards by their extremely resilient rock-like shell, which makes them impervious to all types of gunfire. The Locust appear to use them as a food source, as several butchered corpses are found in the Locust capital of Nexus.
At least three of these exist and are worshiped by the Locust as their gods as part of the Trinity of Worms. These creatures are giant worms that are 8-10 miles long and created the Hollows by moving around underground. Eventually they all entered hibernation, but one was awakened by the Lightmass Bomb and was used to sink three COG cities before it was killed by Delta-One during Operation: Hollow Storm.
These gigantic centipedes originated in the Hollow but have been sighted on the surface after the Hollow's flooding by Jacinto's sinking. They are heavily armored and can attack using electricity in their jaws, their tail being the only weak spot. Utilized in the Locust Army, their fate after the destruction of the Horde is unknown.
Unlike other indigenous creatures, Humans were aware of their existence during the Pendulum Wars. This suggests they are native to the Outer Hollows, but they are known to have survived the Lightmass Bombing so it is possible they also live in the Inner Hollows. They are not part of the Locust Horde but they are known to attack humans due to their predatory nature.
Kryll were highly aggressive flying pack creatures native to the Outer Hollow. They do not inhabit the Inner Hollows but after E-Day they began hunting on the surface of Sera. The species was obliterated by the Lightmass Bombing and they are now extinct.
These tiny purple beetles crawl across the floor of the Inner Hollow in little groups. They are harmless and usually run away when anything comes too close. They can be killed by stepping on them or shooting them.
These strange crane-like bats can be seen flying around the Inner Hollows. They can easily be confused with the Kryll, but they differ as they inhabit the brightly lit areas of the Inner Hollows and do not attack Locust or Gears that they encounter.
These creatures are often seen flying around corpses in the Hollows, presumably feeding and laying their eggs. They're too large to be normal flies and they're also a grayish-purple color. The flies also seem to have four wings.
Fauna and FloraEdit
Although the Outer Hollows were a barren wasteland, the Inner Hollows were much more hospitable and were home to a wide variety of plant life. This included surface-like organisms such as giant mushrooms and tree-like vine plants, to other bizarre flowers, glowing fruits and luminous aquatic fungus unlike anything native to Sera's surface. In the aftermath of the Inner Hollows flooding, it is unknown if any of these plant species survived.
The tunnels that the Locust Horde used to gain access to the surface are scattered and overlapped everywhere underneath the surface, as seen from the JACK bot data. The amount of tunnels seems to be very massive, numbering of about 40 and more, each tunnel being about 100 feet or more/less in length. Some tunnels are wide enough to have over six Corpsers crawling in them at the same time. It is apparent that the larger tunnels were dug by the Riftworm, and the smaller tunnels were made by the Corpsers.