Police in Moscow have discovered what they are calling an "underground town" housing illegal immigrants from Central Asia in a Soviet-era bomb shelter in the west of the city.
The discovery was made by police and agents from the FSB security agency and Federal Migration Service.
The underground area was guarded by a four-metre-high [13 feet] concrete wall and barbed wire, said Andrei Mishel of the Russia's ministry of the interior.
It housed 110 men and women.
"The living areas were fitted with bathrooms, bedrooms and even prayer rooms," Mr Mishal added.
Similar subterranean living quarters were also uncovered in February under an official delegation room in the capital's Kievsky railway station. The people found there were also described as illegal immigrants.
Sewing machine equipment
The shadowy nature of foreign migration to Moscow was underlined at the end of 2010 by Moscow Mayor Sergei Sobyanin, who said that around 250,000 work migrants were registered in the Russian capital, but that the real number was closer to "several million".
Last year around 10,000 illegal migrants were deported from Russia following court decisions against them.
According to official accounts, the migrants living in the underground town were working on producing blades and needles for sewing machines.
The entrance to the underground space was found within the grounds of a closed-off factory, added Mr Mishel, without naming it.
The area houses a company specialising in the production of blades, needles and safety pins, and there is also a military radio-technology factory producing S-300 and S-400 missile systems nearby.
It is thought most of the underground inhabitants are now facing deportation, and a further 16 of them will be investigated for alleged criminal activity.
The owner of the factory might also face criminal action.
A foreign worker in Russia in breach of the Migration Act can be fined up to 5,000 roubles ($177; £108) - and the employer can face a fine of 800,000 roubles for each worker.
At the end of February Moscow police uncovered a number of illegal immigrants under Kievsky station.
It is reported that they were working for a company in charge of cleaning the station area, but the migrants had not been cleared to work. As a result, more than 30 cases of administrative violations were brought against the company.
In March, the city's Federal Migration Service unearthed an underground sausage factory, where more than 30 immigrants from Tajikistan and Moldova were found.
They were not officially registered in the city and therefore lived in the factory itself, sleeping on wooden beds.
The issue of illegal migrants in Moscow remains extremely divisive.
In March, Mayor Sobyanin announced that when hiring employees, Muscovite candidates should be treated preferentially, followed by Russians from outside the capital. Only then should foreigners be considered.
According to Moscow's Central Department of Internal Affairs, migrants are responsible for around 70% of crimes committed in the city.
But Moscow Federal Migration Service representative Mikhail Tyurkin said "according to official statistics foreigners in Russia are only only to blame for around 3.5% of crimes committed".
Recently Aleksandr Bastrykin, of Russia's Investigative Committee, said this criminality was attributable to the conditions the migrants live in.