  A crucial diagnostic of the \Lambda CDM cosmological model is the integrated
Sachs-Wolfe (ISW) effect of large-scale structure on the cosmic microwave
background (CMB). The ISW imprint of superstructures of size \sim100\;h^{-1}
Mpc at redshift $z\sim0.5$ has been detected with $>4\sigma$ significance,
however it has been noted that the signal is much larger than expected. We
revisit the calculation using linear theory predictions in \Lambda CDM
cosmology for the number density of superstructures and their radial density
profile, and take possible selection effects into account. While our expected
signal is larger than previous estimates, it is still inconsistent by
$>3\sigma$ with the observation. If the observed signal is indeed due to the
ISW effect then huge, extremely underdense voids are far more common in the
observed universe than predicted by \Lambda CDM.
