Hollow World

The Hollow World (虚の世界, kyo no sekai) is the main setting of huke's artwork, and the secondary setting of the story of the Black★Rock Shooter OVA and Black★Rock Shooter 2012 anime. Black★Rock Shooter and other characters live in this realm, and the bizarre landscape is a witness to constant fighting between its denizens.

Names
The world depicted in the 2012 TV Anime has several known names:
 * "Another World", "Other World" (裏世界, ura sekai)
 * "Imaginary World" (虚の世界, kyo no sekai)
 * "Hollow World" (虚の世界, kyo no sekai)

There is no known name for the world depicted in the OVA; it is only referred to as "a world other than the one Mato and others are in", and therefore its name is only conjectural.

Black Gold Saw's Battlefield
It is only shown to be a large expanse of land covered by grotesque features of land surrounded by an endless sea. From inside, it is composed of a variety of structures, with most buildings bearing a dilapidated, post-apocalyptic, and even desolate look to them. A striking feature of the structures erected in this realm is its dominant theme of checkered tiles in most of the floors and walls depicted.

Black Gold Saw and Black★Rock Shooter fought here in the beginning of the OVA, where the former was victorious against the latter.

Dead Master's Lair
Dead Master stays inside a structure similar to a cathedral complex located in the middle of a large depression, surrounded with winding passageways from outside. The area is surrounded by a large cemetery, which is shrouded in a thick mist.

Sky Gates
Black★Rock Shooter starts here at the beginning of the anime. The area matches the decor of the Otherworld in general and has pointed, deserted stones everywhere.

Black★Rock Shooter was attacked by a huge King Saw that shot out of the ground from Black Gold Saw's giant eye, which was shortly defeated by Black★Rock Shooter.

Chariot's Land
Chariot seems to reside here. The pink-tinted area (rather than yellow-tinted) is full of robotic, toy-like creatures that have claws and drills. The background resembles a toy area with baby doll faces and colored blocks.

Inside is a house that floats in the air, suspended by strange string-like objects. Dead Master was dragged here by what was presumably Chariot's minion bugs and chained. Only a single doll covered this up.

Chariot and Black★Rock Shooter fought here until Black Gold Saw intervened and made a fissure, separating Dead Master from Black★Rock Shooter.

Black Gold Saw's Battlefield
A grey-tinted area (rather than red-tinted), which seems to have ruined Greek-style columns, and multiple chains spread around.

Black★Gold Saw created the strange hooded figures here, and later fought Insane Black★Rock Shooter here using the same hooded figures.

Grey Area
A separate, grey-tinted area which is a vast expanse of land with chains in the sky, and a large hole leading to an abyss where many bodies of dead other selves lay in.

Strength was shown dragging the beheaded body and head of Chariot here, then throwing her into the abyss. She later dragged the body of Dead Master away from Insane Black★Rock Shooter and Black★Gold Saw's fight, presumably to bring her to this area.

Dead Master's Lair
The strange, green-tinted place where Dead Master was chained might be her lair. In that place, there lies a hidden throne where the Dead Master sits for the duration of time while Black★Rock Shooter fought her skeletal minions. Skeletal columns and Greek-style pillars are scattered about and the entire battlefield has a green tinge.

Covering the floor of some parts of the battlefield is a strange, greenish liquid.

Unknown Battlefield
Until Dead Master was killed by Black★Rock Shooter, this area was Dead Master's lair. The trauma of killing Dead Master transformed Black★Rock Shooter into Insane Black★Rock Shooter, as well as transforming the area into a purple-tinted area filled with spikes and cannon-like objects, possibly meaning this area belongs to Insane Black★Rock Shooter. This is supported by the fact that the area bears an uncanny resemblance to the normal Black★Rock Shooter's realm.

Black★Gold Saw is shown fighting with Insane Black★Rock Shooter here, eventually moving their fight to Black★Gold Saw's own battlefield.

Strength's Battlefield
Strength's field is like that of an orange rubix cube surrounded in gigantic barbed wire. She can easily change her position on the cube by slamming her fists in one of the blocks. She can easily move through the field from jumping or climbing from block to block. It also seems to react to her emotions for when she starts to go insane the field is engulfed in flames.

After saving Black Gold Saw in her own battlefield, Strength fought Insane Black★Rock Shooter here.

Black★Rock Shooter's Battlefield
According to Mato and Insane Black★Rock Shooter, this is Black★Rock Shooter's world. However, she is never shown to actually reside here. The world is a wide, empty space with some ruins embedded in the floor. The ground has the typical checkerboard pattern of most locations in the Otherworld, but with a blueish tint. The entire battlefield seems to be surrounded by a black-blue rocky material that prevents those inside it from seeing the outside world. This battlefield resembles a cavern or a large hall. Unlike most of the other battlefields, it is practically empty except for some large, spiky objects stuck in the ground.

Despite Black★Rock Shooter transforming into Insane Black★Rock Shooter, Mato finds a ★Rock Cannon embedded in the ground here.

Mato Kuroi (as Black★Rock Shooter) fights Insane Black★Rock Shooter here. During the battle, stars begin to appear in the "sky" of the battlefield and eventually the worlds of all the different "other selves" combine with the battlefield, allowing Mato to summon a giant ★Rock Cannon, which she uses to defeat Insane Black★Rock Shooter, and destroy the wall of the battlefield, revealing white space behind it.

Trivia

 * While in the 2012 anime, the Otherworld is primarily referred to in terms of "the other world" or "that other world", the website occasionally uses the term "imaginary world" (虚の世界 kyo no sekai). However, since it always appears in quotes, it can be interpreted as a temporary name, and thus the true name of this world remains undetermined.
 * In the 2012 anime, it is heavily implied to be the "world of many colors" mentioned in the The Tiny Bird & The Colors book.
 * It can be implied that the design of each world represents a certain emotion that a certain main character is trapped in. For example, Strength's world represented by a rubik's cube in motion of being solved, implying one being trapped and unable to escape. Black Gold Saw's world implies desperation and sadness, Chariot's world implies desire and stubbornness, and Dead Master's world implies despair and rejection. Black★Rock Shooter's world while serving as an antithesis represents a blinded view as well as the concept of absorbing all the negative emotions.
 * The merging of the worlds as seen in the anime represents the absorbing and integrating of all emotions into one world, hence giving it completion.
 * In the OVA, some places of the Otherworld that Black★Rock Shooter walks through resemble certain scenes from the Black★Rock Shooter PV.