The one-way frontier around a black hole is the event horizon from which nothing can escape.  Close to it, strong effects of gravity and exotic physics are expected. The most massive black holes are at the centre of distant quasars, which cannot be resolved by telescopes. Fortunately, in some quasars aligned with an intervening galaxy, gravitational microlensing can magnify very small regions within the quasar allowing horizon scale resolution. We have used three events of gravitational microlensing that scan the source of a lensed quasar (Einstein Cross) to resolve a structure, affected by relativistic effects, located at three times the horizon radius, likely the inner rim of the disk of in spiralling matter that surrounds the black hole. In the future, with massive monitoring programs (with the Large Synoptic Survey Telescope, for instance) the possibility of detecting high magnification microlensing events can be extended to thousands of quasars (i.e. hundreds of high magnification microlensing events detected) with a huge improvement in statistical and evolutive (currently known lensed quasars are distributed in 0.5<z<5 redshift) studies of supermassive black holes and accretion discs.