Extreme BL Lacs (EHBL) are a subclass of blazars with unique properties, such as synchrotron peaks in the hard X-ray band and extremely hard TeV spectra peaking above 10 TeV. Their exceptional properties make them interesting sources for current Cherenkov telescopes and a major topic for the upcoming Cherenkov Telescope Array (CTA), the next-generation ground-based γ-ray observatory. In particular, quite interesting is the possibility that, despite the strong γ-γ absorption, EHBL will be detected above 10 TeV. We explore the possibility that photon–axion-like particle oscillations give rise to an effective decrease of the opacity.