Beta Acid

Unidentified Substance 4 is a highly corrosive blue-green liquid found in Mother Brain's chamber after Tourian is destroyed in Metroid: Zero Mission, replacing the lava that was there before. It is possible that this is in fact US 1 but this fact cannot be confirmed due to Metroid II's black-and-white color scheme. It is severely dangerous to step into even if Samus is wearing the Gravity Suit. This substance is not present in Super Metroid, which could suggest that it dried up or drained from the room.

The same substance is also present aboard the Biologic Space Laboratories research station, in two corridors in the Tourian-like area of Sector 1, one of them being the room before Neo Ridley's chamber. Like in Zero Mission, the substance is harmful even to the Gravity Suit. It is possible that the scientists at the BSL station had included it to enhance the accuracy of their duplicate SR388 ecosystem, if it is indeed US1.

Lava is preasant in some areas of Sector 1, but Samus does not encounter any other material that could pass for US 1 within the Federation's otherwise faithful simulated environment.

This substance, being corrosive and green, may be yet another reference to the Alien film franchise, as the Aliens in the franchise bled corrosive green blood.