  We present the nuclear matrix element for the neutrinoless double-beta decay
of $^{48}$Ca based on large-scale shell-model calculations including two
harmonic oscillator shells ($sd$ and $pf$ shells). The excitation spectra of
$^{48}$Ca and $^{48}$Ti, and the two-neutrino double-beta decay of $^{48}$Ca
are reproduced in good agreement to experiment. We find that the neutrinoless
double-beta decay nuclear matrix element is enhanced by about 30\% compared to
$pf$-shell calculations. This reduces the decay lifetime by almost a factor of
two. The matrix-element increase is mostly due to pairing correlations
associated with cross-shell $sd$-$pf$ excitations. We also investigate possible
implications for heavier neutrinoless double-beta decay candidates.
