Scientists on the DZero collaboration at the U.S. Department of Energy’s Fermilab have discovered a new particle—the latest member to be added to the exotic species of particle known as tetraquarks.

Quarks are point-like particles that typically come in packages of two or three, the most familiar of which are the proton and neutron (each is made of three quarks). There are six types, or “flavors,” of quark to choose from: up, down, strange, charm, bottom and top. Each of these also has an antimatter counterpart.

Over the last 60 years, scientists have observed hundreds of combinations of quark duos and trios.

In 2003 scientists on the Belle experiment in Japan reported the first evidence of quarks hanging out as a foursome, forming a tetraquark. Since then physicists have glimpsed a handful of different tetraquark candidates, including now the recent discovery by DZero—the first observed to contain four different quark flavors.

DZero is one of two experiments at Fermilab’s Tevatron collider. Although the Tevatron was retired in 2011, the experiments continue to analyze billions of previously recorded events from its collisions.

As is the case with many discoveries, the tetraquark observation came as a surprise when DZero scientists first saw hints in July 2015 of the new particle, called X(5568), named for its mass—5568 megaelectronvolts.

“At first, we didn’t believe it was a new particle,” says DZero co-spokesperson Dmitri Denisov. “Only after we performed multiple cross-checks did we start to believe that the signal we saw could not be explained by backgrounds or known processes, but was evidence of a new particle.”