The COMPASS experiment. (Image: CERN) Protons are one of the main building blocks of the visible universe. Together with neutrons, they make up the nuclei of every atom. Yet, several questions loom about some of the proton’s most fundamental properties, such as its size, internal structure and intrinsic spin. In December 2020, the CERN Research Board approved the first phase (“phase-1”) of a new experiment that will help settle some of these questions. AMBER, or Apparatus for Meson and Baryon Experimental Research, will be the next-generation successor of the Laboratory’s COMPASS experiment. COMPASS receives particle beams from CERN’s Super Proton Synchrotron and directs them onto various targets to study how quarks and gluons form hadrons (such as protons, pions and kaons) and give these composite particles their distinctive properties. Using this approach, COMPASS has obtained many important results, including several results linked to the proton’s spin structure and a measurement of the pion’s polarisability; the polarisability of a hadron is the degree to which its constituent positive and negative electric charges can be separated in an electric field.