The muon collider: expected physics, technological solutions, and the prospect of a 21 km ring at the UNK site
A multi-TeV muon collider has emerged as one of the most compelling options for the post-LHC energy frontier. Because the muon is an elementary particle that radiates $\sim\!10^9$ times less synchrotron power than an electron of the same energy, a circular muon collider delivers the full beam energy to the hard collision in a remarkably compact ring, combining the energy reach of a $100$~TeV-class proton machine with the clean final states of a lepton collider. This review summarizes the expected physics results -- Higgs couplings and self-couplings, electroweak and vector-boson-fusion processes, the top quark, a broad beyond-the-Standard Model programme, and a near-complete closure of the thermal window for electroweak WIMP dark matter -- and the status of the enabling technologies, emphasizing for each challenge how it is being solved and where the solution is documented: muon production, ionization cooling (demonstrated in the transverse plane by MICE), rapid acceleration, high-field HTS magnets, the machine-detector interface, and the neutrino-flux constraint. Building on this foundation, this review examines the prospect of housing a muon collider in the existing 21~km UNK tunnel near Protvino, where the magnetic-rigidity relation maps realistic arc fields onto a centre-of-mass energy of order $10$-$20$ TeV -- reaching the IMCC 10~TeV reference with conventional dipoles and exceeding it with high-temperature-superconductor dipoles. It closes with a discussion of non-collider applications of the intense muon beams such a facility would develop -- muon-catalyzed fusion, muography, muonic-atom isotopic analysis, and muon spin spectroscopy -- several with a long tradition in the Russian physics institutes.