Physicists at the KEK particle physics laboratory in Tsukuba, Japan, have found a sub-atomic particle they cannot explain using current theories. Named X(3872), it is found among the decay products of bottom mesons at the KEK "bottom factory". Its mass of 3.9 GeV is not explainable as from a quark-antiquark composite as with ordinary mesons, and may be a first glimpse of a four-quark state.