It appears Bob Lazar claimed in the 90s that when we finally discovered and synthesized Element 115 it would within a "stable island of elements" on the periodic table.

Bob Lazar UNSEEN Interview with Don Garlits (1997) Around the 33:40 minute mark: "The fuel is an element that we don't have here on earth knows as element 115. Uh it is also something we can't synthesize in an accelerator. Um, there are heavier elements that we do synthesize like plutonium and things along those lines and generally they have a half-life which means the disintegrate very rapidly and the higher you go up on the periodic chart the faster these things disintegrate. However we hav...