Some of the tunnels reportedly even have rooms and toilets.
The Kurdish YPG, a contingent of the US-backed forces fighting ISIS in Syria, released a video on Tuesday showing the underground tunnels that ISIS digs to launch sneak attacks.
The video shows two rather large tunnels inside a captured, bombed-out mosque, from which the YPG claim that ISIS had been using.
"The barbaric group, aware of the YPG's sensitivity towards people's places of worship, and other historic sites, has been using [mosques] as bases to delay the liberation of Raqqa," text in the YPG video reads.
ISIS has been known to use such tunnels in Iraq and Syria not only for sneak attacks, which the militants reportedly paid civilians $2 per day to dig, are also used for moving supplies, housing ISIS fighters and laying booby traps.
Former ISIS fighters have reportedly said that some of the tunnels are extremely complex, some even containing rooms, toilets and medical facilities.
A YPG commander recently said there are about 700 to 1,000 ISIS fighters left in Raqqa, and that the battle should be over in about 2 months.
This older Fox News video shows how intricate the tunnels can get: