Machine Island is the main setting for the Mazinkaiser SKL OVA and volume 2 of Mazinkaiser SKL Versus. It is a floating island that was originally the site of a gravity experiment. However the device was too powerful to completely control and isolated the island from the outside, enveloping it within by a powerful energy field known as the Gravity Curtain.
Machine Island is kept in tact by the Gravity Curtain which also doubles as a barrier, making sure no one goes in or out of the island. It also houses a vast amount of resources especially around areas close to the three generators of the Gravity Curtain. The manga mentions a series of robot factories around the island that continuously produces robots. These areas became dominated by the three armies that inhabit the island: the Kiba and Garan Armies, and the Hachiryokaku. The resources allow them to build large fortresses and robots for battle or protection.
While it is unknown about the history of all of the inhabitants, it is easy to say from the area that they are various people such as survivors from a crashed aircraft, citizens of the area, etc. to Machine Island. Ever since the end of a great war, people were imprisoned in the island and conflicts arose for control of the island especially with the robots that are continuously being produced. Overtime, three charismatic people formed their own armies to survive and take control of the island or defend themselves from threats. Many battles were fought on the island for dominance. Overtime, the Gravity Curtain started to overload and the Skull Force was deployed to power it down. Only three members survived and managed to defeat the leaders of the Kiba and Garan armies until the Gravity Curtain started to go out of control as the Iron Kaiser was reactivated. After a lengthy fight, the Gravity Curtain and Iron Kaiser were destroyed by the Mazinkaiser SKL, while the rest of the fortresses and island was destroyed the Hachiryokaku survived and it was assumed that the survivors were either brought back to the mainland such as some of the Psycho Gear pilots or remained on the island.