During 1949, the preceding two elements, berkelium (97) and californium (98) were synthesized in minute amounts. It was then clear that the synthesis of elements beyond californium could be achieved by two methods: exposing heavy elements to a very high neutron flux, or bombarding them with heavier ions, such as carbon or nitrogen.

The Materials Testing Reactor in Idaho came into operation during 1952, and provided a neutron flux an order of magnitude higher than previously available. At the same time, techniques for accelerating useful beams of heavy ions were being developed in several laboratories. These developments ensured the eventual synthesis of elements 99 and 100, but the first observation of these elements came unexpectedly, from a quite unrelated experiment - the explosion of the first thermonuclear bomb in the Pacific during November 1952.

Samples of debris were collected by drone aircraft flying through the cloud and analyzed. Among the new activities detected were alpha-emitters of 6.6 and 7.1 MeV, indicating that a brief exposition to a very high neutron flux can have similar results to the slow irradiation by heavy elements made on the labs in Idaho. From the analysis of these samples it was possible to identify the source of the alpha particles with that specific energy as the new elements 99 and 100 respectively.

During 1953 and early 1954, while the discovery of elements 99 and 100 was withheld from publication, a group at the Nobel Institute for Physics in Stockholm bombarded U 238 with oxygen-16 ions, and isolated a 30-minute alpha emitter which they tentatively ascribed to a new isotope (250) of element 100, without claiming the discovery of the element.

The full story of the discovery of elements 99 and 100 was finally published in mid-1955 by a group of authors of the laboratories involved.

Element 99 was named einsteinium, in honor of Albert Einstein, and element 100 was named fermium, in honor of Enrico Fermi.