Rhenium (ree-nee-əm)
is a chemical element with the symbol Re and it has an
atomic number of 75. The atomic weight of Rhenium is
186.207. 
Things you might not know about
Rhenium:
- Rhenium was the next-to-last naturally
occurring element to be discovered.
- Rhenium is one of the rarest elements in
Earth's crust with an average concentration of 1 ppb.
- Rhenium is added to high-temperature
superalloys that are used to make jet engine parts, making 70% of
the worldwide rhenium production.
See GC2XAY2 - Hydrogen, for an explanation
about this series of caches.