The Battle for Fallujah: Inside Islamic State's secret tunnels

Fallujah has been under Islamic State (Isis/Daesh) rule for two years, after it was invaded by the militant group in January 2014. In a bid to reclaim the city, Iraqi forces, backed by Shi'ite militias, have begun what has been described as one of the biggest battles ever fought between Isis militants. Tens of thousands of people remain trapped in the city. Confined to Fallujah, residents are being used as 'human shields' and have limited access to water, food and healthcare; many of then are starving to death.

The militant group has managed to counter many of the assaults made on the city by hiding underground. Tunnels hidden beneath the city – where the group's fighters sleep and hide – have played a pivotal role in the militants' counter offensive against Iraqi forces.

ISIS tunnels — A fighter from the Iraqi Shi'ite Badr Organisation holds his rifle in an underground tunnel built by Islamic State fighters on the outskirts of Fallujah, Iraq Thaier Al-Sudani/ Reuters

Daesh use the tunnels to approach, appear and escape from the front line, in order to reduce their casualties. According to Hashid Shaabi, an Iraqi coalition of mostly Shi'ite Muslim paramilitary groups, the longest tunnel discovered surrounded a brick factory.

The findings of the Fallujah tunnels follow those discovered in Sinjar, Nineveh province back in November 2015. Found by Kurdish forces, these tunnels were filled with clothing, medical supplies and bomb making equipment. According to Foreign Affairs, the tunnels lead into houses, situated above ground, enabling militants to 'move through the city undetected'.

Fallujah is one of the last IS strongholds in Iraq. The militants also hold Mosul, the second largest city.