A heavy-ion collision recorded by CMS in 2011 (Image: Tom McCauley/CMS/CERN) For a few weeks each year of operation, instead of colliding protons, the Large Hadron Collider (LHC) collides nuclei of heavy elements (“heavy ions”). These heavy-ion collisions allow researchers to recreate in the laboratory conditions that existed in the very early universe, such as the soup-like state of free quarks and gluons known as the quark–gluon plasma. Now, for the first time, the Compact Muon Solenoid (CMS) collaboration at CERN is making its heavy-ion data publicly available via the CERN Open Data portal. The open data are available in the same high-quality format used by the CMS scientists to publish their research papers. The data are accompanied by the software that is needed to analyse them and by analysis examples. Previous releases of CMS open data have been used not only in education but also to perform novel research. CMS is hopeful that communities of professional researchers and amateur enthusiasts as well as educators and students at all levels will put the heavy-ion data to similar use.