  The ABC effect -- a long-standing puzzle in double-pionic fusion -- has been
reexamined by the first exclusive and kinematically complete measurements of
solid statistics for the fusion reactions $pn \to d\pi^0\pi^0$, $pd \to
^3$He$\pi\pi$ and $dd \to ^4$He$\pi\pi$ using the WASA detector, first at
CELSIUS and recently at COSY -- the latter with a statistics increased by
another two orders of magnitude. In all cases we observe a huge low-mass
enhancement in the $\pi\pi$-invariant mass accompanied by a pronounced
$\Delta\Delta$ excitation. For the most basic fusion reaction, the $pn \to
d\pi^0\pi^0$ reaction, we observe in addition a very pronounced resonance-like
energy dependence in the total cross section with a maximum 90 MeV below the
$\Delta\Delta$ mass and a width of only 50 MeV, which is five times smaller
than expected from a conventional $t$-channel $\Delta\Delta$ excitation. This
reveals the ABC effect to be the consequence of a s-channel resonance with the
formfactor of this dibaryonic state being reflected in the low-mass enhancement
of the $\pi\pi$-invariant mass. From the fusion reactions to $^3$He and $^4$He
we learn that this resonance is robust enough to survive even in nuclei.
