Residents of Odesa are preparing to use the extensive underground tunnels in the southern Ukrainian port city as shelters following Russia's invasion of the country.

The huge network of long tunnels, known as the Odesa Catacombs, was built in the 19th century to serve limestone quarries.

Citizens used the tunnels during World War Two as a stronghold of their resistance movement against Nazi Germany's invasion. After the war, it is believed that the former Soviet Union's intelligence agency used them. Part of the structure has been maintained as a tourist attraction in recent years.

Odesa residents have begun to use the tunnels as shelters again since the start of the Russian invasion in February.

Several dozen families have already evacuated to a section about 20 meters belowground. The date "March 1, 2022," is written on a wall, indicating when someone took shelter.

Citizens are bringing water, food, chairs, mattresses and other supplies into the tunnels. Volunteers are installing electric lighting.

One volunteer said he hopes to raise donations to buy generators to improve the living environment. He added that everyone is trying to do what he or she can.