I'm sorry but this is WRONG.
Element 115 is a real element in REAL life.
It came from a meteorite that was known as the Tungusten (misspelled) event. The meteorite actually landed in an area that looks VERY similar to the map Shi No Numa.
From that point on, everything that happened to samples taken, was all rumor and legend and old stories that people thought of, or claimed to have happened.
I don't remember who got to the meteor first to extract the element, or how much was extracted. But it did eventually end up in the Factory of Der Riese. There the radiation from the meteor was liquified into a physical (yet liquid) component. From then on they started experimenting with the element. Eventually, so many tests later, they found that the element begun reanimating dead body cells.
So in a sense, zombies originated from the Factory.
UNLESS, from the moment the meteor landed, the radiation soaked itself into the ground and immediately begun bringing the dead back to life.
Have you ever thought why every round are there more and more zombies?
It's because since the zombie is full of the element and its radiation continues to emit from the body even after death, the radiation will begin to soak into the ground once again and continue infecting the dead bodies, which explains why you kill 10 zombies, 25 more show up.
This guys good...