  We investigate the dynamics of two miscible superfluids experiencing fast
counterflow in a narrow channel. The superfluids are formed by two
distinguishable components of a trapped dilute-gas Bose-Einstein condensate
(BEC). The onset of counterflow-induced modulational instability throughout the
cloud is observed and shown to lead to the proliferation of dark-dark vector
solitons. These solitons, which we observe for the first time in a BEC, do not
exist in single-component systems, exhibit intriguing beating dynamics and can
experience a transverse instability leading to vortex line structures.
Experimental results and multi-dimensional numerical simulations are presented.
