A group of 32 people, members of the so-called True Russian Orthodox Church, moved into the shelter, which contains wells, a kitchen, monastic cells and other facilities, last week, threatening to set themselves on fire if police tried to force them out.
The cult, led by Father Pyotr, a 43-year-old diagnosed schizophrenic, expect Judgement Day to arrive in May 2008.
Alexander Dvorkin, a Russian expert on sects said, "The situation is on a knife edge, and anything could happen at any moment," adding that the group also contained four children.
Dvorkin also said that totalitarian sects were common in Russia, and that "control over their members is absolute, and anything that comes into the heads of their leaders has a direct effect on the entire group."
He also added that the group in the Penza Region was similar in outlook to those pseudo-Russian Orthodox groups calling for the canonization of Stalin and Ivan the Terrible.
Meanwhile, a Russian tabloid, Tvoi Den, cited a police source as saying, "If talks don't work, we will fire sleeping gas into the tunnel. The gas is not harmful for human health, however."
It was not immediately clear why the group had decided to go underground this month if they do not expect the apocalypse to arrive until May of next year.
One of the most well-known sects in Russia has its base near the southern Siberian town of Abakan, where thousands of people, both Russian and foreign, worship a former Russian provincial traffic policeman, Sergei Torop, as the second coming of Christ.