This spurred Hahn and Meitner, the discoverers of the most stable isotope of protactinium, to conduct a four-year-long investigation into the process with their colleague Strassmann. After much hard work and many discoveries, they determined that what they were observing was fission, and that the new elements that Fermi had found were fission products. Their work overturned long-held beliefs in physics and paved the way for the discovery of the real elements 93 (neptunium) and 94 (plutonium), for the discovery of fission in other elements, and for the determination of the role of the uranium-235 isotope in that of uranium. Niels Bohr and John Wheeler reworked the liquid drop model to explain the mechanism of fission.