  Gravitational microlensing has been employed to identify massive halo objects
by their amplification of distant sources; MACHO searches have studied event
times $2 h \lesssim t_0 \lesssim 2 y$ corresponding masses in the range
$10^{-6} M_{\odot} \lesssim M \lesssim 100 M_{\odot}$. We suggest that larger
masses up to $10^6 M_{\odot}$ are also of considerable interest. It has not
been excluded that there is a significant number of halo black holes with such
high masses as suggested by cosmological entropy considerations and potentially
detectable by high longevity microlensing events.
