http://www.kerryr.net/pioneers/tesla.htmexcerpt:
At the age of 81, Tesla challenged Einstein's theory of relativity, announcing that he was working on a dynamic theory of gravity that would do away with the calculation of space curvature.
The theory was never published, but a similar theory involving gravity waves - developed in the mid-1990s - is used in the study of plasma cosmology (which explains properties of energy and the structure of the universe by studying the electromagnetic effects of plasma).
In 1943, at the age of 86, Tesla offered his much-vaunted 'Death Ray' to the US War Department. There's some confusion on whether the 'ray' consisted of laser or particle beams (both of which Tesla had been mulling over for years), or if a working prototype had been developed.
Tesla couldn't share the details. In a coincidence that raises the hackles of conspiracy theorists to this day, Tesla died sometime between that evening and when his body was found three days later.
Nikola Tesla - one of the greatest geniuses of any age - died of heart failure, alone and destitute in a shabby hotel room.
And then the FBI nicked everything he owned.