  The polar phase of $^3$He, which is topological spin-triplet superfluid with
the Dirac nodal line in the spectrum of Bogolubov quasiparticles, has been
recently stabilized in a nanoconfined geometry. We pump magnetic excitations
(magnons) into the sample of polar phase and observe how they form a
Bose-Einstein condensate, revealed by coherent precession of the magnetization
of the sample. Spin superfluidity, which supports this coherence, is associated
with the spontaneous breaking of U(1) symmetry by the phase of precession. We
observe the corresponding Nambu-Goldstone boson and measure its mass emerging
when applied rf field violates the U(1) symmetry explicitly. We suggest that
the magnon BEC in the polar phase is a powerful probe for topological objects
such as vortices and solitons and topological nodes in the fermionic spectrum.
