Invisible platform

Unidentified Substance 6 (Metroid Prime) is a peculiar type of stone encountered in the Tallon Overworld on Tallon IV, in the Root Cave and Great Tree Hall. The substance is completely invisible to both the naked eye and to thermal imaging, although the X-Ray Visor will reveal it to be a porous, light-colored rock. Large pieces of the substance also possess the unusual ability to levitate without any means of support- whatever force holds them aloft seems to be quite powerful, as the rocks are capable of holding Samus's weight without shifting from their original positions to any noticable degree.

This allows them to serve as invisible platforms which allow Samus to reach Missile Expansions hidden near the top of both rooms. Generally the X-Ray Visor is required to see and thus navigate them, but it is possible to acquire the expansions without it by searching for areas where the rain seems to stop in midair.

Trivia

 * It is unknown how exactly this material is able to seemingly defy gravity. It is possible that it possesses extremely pronounced diamagnetic propertes, which would cause it to be repelled by Tallon IV's planetary magneitc field.
 * However, it is also possible that US 6's properties stem from Phazon- a Tallonian creature known as Thardus consisted of "animated" stone bound together with Phazon energy.
 * Much larger floating rocks can be observed around and below the Cradle and Impact Crater, also in the Tallon Overworld. However, these rocks are visible to the Combat and Thermal visors as well as the X-Ray Visor.
 * In Super Metroid, some rooms in Crateria feature what appear to be floating rock platforms, although these too are visible to the naked eye and appear to have a much rougher texture than the Tallonian variety.