Massive rainstorm struck Moscow over the weekend, with 40% of the monthly average precipitation falling down on Russia’s capital in an hour, causing localized flooding in some areas.

Despite dangerous weather conditions, a group of tourists still embarked on an illegal tour of sewage network beneath central Moscow, passed Sunday, during the storm.

According to the local reports, about twenty adventurers had initially signed up for an excursion of the Neglinnaya River, which flows in underground tunnels through central Moscow, and which was scheduled to take place that day.

While most of the registered explorers cancelled their tour due to the bad weather forecast, eight people still descended into the sewers.

The group was apparently underground when the rain suddenly intensified and some tour group members ended up being swept away by rising water.

As a result, three people have died and others remain missing and also feared dead after being swept away by an underground water surge.

Yesterday, the body of a 15-year-old girl was recovered from the Moskva River. The body of a 17-year-old boy was found in the water in the same area later that day.

Another body, that of a male, had been fished out from a sewage collector near the capital’s Paveletsky train station. The fate of the other participants currently remains unknown.

The operators of the Neglinnaya River tours have been offering their services online for about $95 per person. As of right now, all of their upcoming tours have been canceled, according to their website.

Russian law enforcement authorities have opened a criminal case, investigating provision of commercial services that do not meet safety standards, after the deadly sewers tour.

If convicted, the tour operators could face a punishment of up to ten years in prison.