When Marawi residents were finally allowed to check on their houses about seven months after the dust of battle had settled, they saw holes in the walls and tunnels underneath. This was in the so-called "most affected areas" or ground zero, which experienced the heaviest and most devastating bombardment in the 2017 Marawi siege.

Initially, the residents did not know the significance of the holes and tunnels. Later, they learned that it was to facilitate the movement of the rebels from one building to another without being exposed on the roads that were being monitored using satellites and drones, courtesy of foreign countries. They could evade the modern penetrating binoculars and sophisticated war weapons that honed in on body heat.

Tunnel or subterranean warfare is a feature of modern urban wars that we saw in the Vietnam war, the Syrian civil war, and the campaigns against the terrorist ISIS and similar groups. We are seeing it now in the Israel-Hamas conflict.

In the Marawi war, the Maute-Abu Sayyaf Group took advantage of the labyrinth of sewers and canals that crisscrossed buildings underground. This could have been the reason for the protracted fighting, which brought government forces aided by modern technology and intelligence provided by countries like the United States five months to quell even if the rebels were clearly outnumbered and outgunned.

In the basement of the Bato Ali Mosque, which was used as a temporary shelter cum headquarters of the rebels, there was a covered canal going down towards the Dansalan area where the wet market or padian was situated and where the other rebel forces were holding out.  It was in this area that sniper bullets killed rebel leaders Omar Maute and Istilon Hapilon before dawn on 16 October 2017.

The current war in the Middle East will not be a walk in the park for the Israeli forces. How do we deal with the network of tunnels that were dug before the wars between Israel and the Arabs that are now being used by Hamas? There are more than a thousand subterranean tunnels in Gaza alone. They connect to the countries surrounding Gaza. They were initially used by smugglers of illegal drugs and by human traffickers. They are now referred to as "terror tunnels." They were dug so deep that even the most destructive bombs of Israel cannot destroy them. The Palestinians built tunnels beneath hospitals, schools, churches and mosques, which are not likely to be bombed. They were used to pull off the surprise attack on Israel on 7 October.