  Black hole mass scaling relations suggest that extremely massive black holes
(EMBHs) with $M_\mathrm{BH}\ge10^{9.4}\,M_{\odot}$ are found in the most
massive galaxies with $M_\mathrm{star}\ge10^{11.6}\,M_{\odot}$, which are
commonly found in dense environments, like galaxy clusters. Therefore, one can
expect that there is a close connection between active EMBHs and dense
environments. Here, we study the environments of 9461 galaxies and 2943 quasars
at $0.24 \le z \le 0.40$, among which 52 are extremely massive quasars with
$\log(M_\mathrm{BH}/M_{\odot}) \ge 9.4$, using Sloan Digital Sky Survey and MMT
Hectospec data. We find that, on average, both massive quasars and massive
galaxies reside in environments more than $\sim2$ times as dense as those of
their less massive counterparts with $\log(M_\mathrm{BH}/M_{\odot}) \le 9.0$.
However, massive quasars reside in environments about half as dense as inactive
galaxies with $\log(M_\mathrm{BH}/M_{\odot}) \ge 9.4$, and only about one third
of massive quasars are found in galaxy clusters, while about two thirds of
massive galaxies reside in such clusters. This indicates that massive galaxies
are a much better signpost for galaxy clusters than massive quasars. The
prevalence of massive quasars in moderate to low density environments is
puzzling, considering that several simulation results show that these quasars
appear to prefer dense environments. Several possible reasons for this
discrepancy are discussed, although further investigation is needed to obtain a
definite explanation.
