If you break a magnet in two, you don't get a north half and a south half - you get two new magnets, each with two poles.
"Monopoles" were famously predicted to exist by physicist Paul Dirac in 1931 - but they have remained elusive.
Now scientists have engineered a synthetic monopole in a quantum system for the first time, allowing its mysterious properties to be explored.
They describe their breakthrough in Nature journal.
"Detecting a natural magnetic monopole would be a revolutionary event comparable to the discovery of the electron," wrote the team from Aalto University, Finland, and Amherst College, US, in their paper.
"[Our work] provides conclusive and long-awaited experimental evidence of the existence of Dirac monopoles.
"It provides an unprecedented opportunity to observe and manipulate these quantum mechanical entities in a controlled environment."
The discovery of magnetic monopoles has been long-awaited as they can help to explain various physical phenomena.
Researchers have hunted for them since Paul Dirac first theorised their quantum-mechanical characteristics in 1931.
He demonstrated that if even a single monopole exists, then all electrical charge must come in discrete packets - which has indeed been demonstrated.
To observe and test them in the lab, scientists engineered a quantum system - the magnetic field of a cloud of rubidium atoms in an unusual state of matter known as a Bose-Einstein condensate.
Using direct imaging, they detected a distinct signature of the Dirac monopole - known as a "Dirac string".
The researchers note that - while other teams have previously made analogues of monopoles, their demonstration is the first in a quantum system which can be tested by experiment.
"This creation of a Dirac monopole is a beautiful demonstration of quantum simulation," said Lindsay LeBlanc, of the University of Alberta, a physicist not involved in the study.
"Although these results offer only an analogy to a magnetic monopole, their compatibility with theory reinforces the expectation that this particle will be detected experimentally.
"As Dirac said in 1931: 'Under these circumstances one would be surprised if Nature had made no use of it'."