Mysterious Cubes

The Mysterious Cube is an object that resembles a cube.

Overview
The cube was first seen 18 years ago during a flashback of Tatsumaki when she was saved by Blast from a monster. Though it is unknown how the cube operates.

Little is known about this cube; however, the cube appears to be extremely heavy, as it was able to get Flashy Flash's arm stuck, sinking into the ground when Saitama dropped it as well as the fact that Manako, a monster capable of lifting fairly large rocks, wasn't even able to move it. When the Monster Association Headquarters was lifted into the air by Tatsumaki, Saitama along with Flashy Flash and Manako saw through a hole where Flashy Flash's arm was stuck and discovered an extremely gigantic humanoid entity that appeared to be in the space. Still, it is unclear whether these unusual things have something to do with the mysterious cube which Saitama threw away or not.

Monster Association Arc
The cube appears for the first time during Tatsumaki's flashback as she recalls the events of 18 years ago when she was saved by Blast from a monster. Here, it is revealed that Blast was holding a black cube when having a conversation with Tatsumaki at the time. After having removed something that sticks Flashy Flash's arm, Saitama finds an object that resembles the mysterious cube which Blast was holding 18 years ago, and throws it away without having any idea what it actually is. Later, when Tatsumaki begins to lift the Monster Association Headquarters into the air, Saitama along with Flashy Flash and Manako discover a hole where Flashy Flash's arm was stuck, they later see through the hole and detect something like an extremely gigantic humanoid entity in a fetal position appears to be in the space.