  There are several new projects to survey the sky with millimetre eyes, the
biggest being Simons Observatory and CMB-S4, in the Southern Hemisphere. The
NIKA2 collaboration has acquired sufficient knowledge to build a large focal
plane KID camera for a 15~m antenna. This would allow covering the whole
Northern Hemisphere in five years at subarcminute resolution and with
milliJansky point-source sensitivity. We describe the main scientific drivers
for such a project: the SZ sky, the high-redshift millimetre Universe and the
interstellar medium in our Galaxy and the nearby galaxies. We also show briefly
the main difficulties (scientific, organisational, technical and financial).
