The cities of Bakhmut and Soledar in eastern Ukraine have been heavily contested for days. A tunnel system under the cities plays a central role. Mercenary boss Yevgeny Prigozhin wants to take this for his “Group Wagner”.

According to President Volodymyr Zelenskyy, the Ukrainian army has to defend itself against massive attacks by Russian forces in the east of the country. The situation on the ground is dramatic. “Everything is completely destroyed. The ground in Soledar is covered with the bodies of the attackers and scarred by explosions,” he said Monday in an evening address.

Soledar and the neighboring town of Bakhmut are part of the Ukrainian line of defense in front of the Sloviansk-Kramatorsk conurbation. From the Russian point of view, capturing the territory would be a significant step towards conquering the entire Donbass. But there’s another reason why the two cities are so important for the coming war: A gigantic tunnel system under the area could offer a decisive advantage for the winner of the battle.

Yevgeny Prigozhin, head of the brutal Russian mercenary group Wagner, is particularly interested in the tunnels, reports the Reuters news agency. The system extends over 200 kilometers through the terrestrial space of the entire region. On Telegram, Prigozhin described the tunnels as a “network of underground cities”, which for him would be the “icing on the cake” in addition to the strategic location.

The tunnels are wide enough to hide tanks and other military vehicles and station large groups of soldiers. The tunnels could be used, for example, to penetrate enemy defense lines underground. Prigozhin also claimed that weapons had been hidden in the mine system since World War I.

The system is composed of various resource-rich salt and gypsum mines that run at a depth of up to 290 meters. Not all tunnels are still actively used for resource extraction. One of the largest chambers has hosted concerts or football matches, and there is even a church whose walls are decorated with detailed salt carvings.

Russian forces have been trying to take Bakhmut and Soledar for five months, but the offensive keeps bogging down. Mercenary chief Prigozhin explained that in Bakhmut “every house is a fortress”. His soldiers have to fight for every single house, sometimes it even takes several weeks. The main problem is the strong defense of the Ukrainian army. He speaks of hundreds of defense lines across the city.

It is not certain how long the fighting in Bakhmut and Soledar will continue. However, British military experts consider a sustained breakthrough by Russian forces in the coming weeks to be unlikely. According to an intelligence update from the Ministry of Defense in London, available to the BBC, Ukraine’s defense line is “very stable”.