"One of the most significant goals of modern astronomy is establishing whether life exists around other stars. The most direct path towards its achievement is the detection and spectroscopic characterization of terrestrial planets orbiting in the habitable zone of their host stars. The ~1000 nearest ultra-cool stars (UCS, spectral type M6 and latter) represent a unique opportunity to reach this goal within the next decade. Due to their small luminosity, their habitable zone is 30-100 times closer than for the Sun, the corresponding orbital periods ranging from one to a few days. Thanks to this proximity, the transits of habitable planets are much more probable and frequent than for Earth-Sun analogs, while the small size of UCS (~1 Jupiter radius) leads to transits deep enough for a ground-based detection, even for sub-Earth sized planets. Furthermore, habitable planets transiting nearby UCS would be amenable for a thorough atmospheric characterization with near-to-come world-class facilities, including the detection of possible biosignatures. Detecting such planets is the goal of SPECULOOS. Its instrumental concept is optimized for the detection of planets of Earth-size and below transiting the nearest Southern UCS. It consists in several robotic 1m-class telescopes equipped with new generation CCD cameras optimized for the near-IR and operating from one of the best astronomical sites of the Southern hemisphere. SPECULOOS will perform the first exploration of the Terra Incognita of planets around UCS, and detect the first terrestrial planets amenable for atmospheric characterization."