Neptune was the Roman god of the sea, but he was sometimes associated with horses because of his Greek counterpart Poseidon, so seapony Silverstream appears here swimming in a pool. The 60-inch cyclotron that was used to first synthesise the element, by Edwin McMillan and Philip Abelson in 1940, lies above. (The element has no significant uses besides being an intermediate in the production of plutonium.)

SVG here.