Particle tunneling through the classically forbidden region beneath a potential barrier is a purely quantum-mechanical effect, described in 1928 by George Gamow to explain the finite probability of radioactive alpha decay of nuclei. The same mechanism, associated with Cooper pair tunneling, underlies the Josephson effect — the dissipationless flow of current through a junction between two superconductors. A finite voltage drop, i.e., the destruction of the superconducting state, occurs at currents greater than a certain critical value. The emergence of a dissipative regime is also possible at currents less than the critical value due to jumps in the macroscopic variable of the phase difference of the superconducting states at the junction, which occur due to both the thermally activated mechanism of overcoming the potential barrier and quantum subbarrier tunneling. Experimental studies of the transition between these two mechanisms in an electric circuit with a Josephson junction were awarded the 2025 Nobel Prize in Physics.