A year into Israel’s fight against Hamas, it has become clear that the military “center of gravity” — the most important element of the conflict — is not the missiles or manpower of the terrorist group. Rather, it is the 400-plus miles of tunnels carved out under the Gaza Strip. From those tunnels, Hamas and its sponsor, Iran, were able to train, equip, organize and launch the horrific attacks of Oct. 7.

The Israel Defense Forces have now publicly released a handbook captured from Hamas in 2019 that details how the group sought to maximize the lethality of capabilities it painstakingly built up underground and out of sight. The group trained forces to fight in the subterranean environment using cover of darkness, night-vision goggles, split-second timing, GPS trackers, elaborate camouflage and protective blast doors. Hamas troops managed to create an entirely different battlefield from the traditional fight above ground.

It is easy to believe that Hamas did so in a burst of unique creativity, and that we are seeing the emergence of a new style of warfare. In fact, the use of tunnels in war has a long history.

The question today is how the advent of cutting-edge technologies can enhance this ancient style of combat, presenting military planners with new challenges around the globe. Where are other deadly tunnel complexes? What should the U.S. and its allies do to prepare?

Sappers have constructed tunnel complexes over the centuries, going back to the ancient Greek and Roman armies. Ironically, recent excavations found that the Jewish rebels of Judea used hundreds of miles of tunnels connecting villages in their revolt against the Romans two millenniums ago. The ancient Chinese and Ottoman Turks both used tunnel complexes, notably in undermining cities under siege.

Tunnels played an important role in World Wars I and II. In the former, for example, Allied troops planted powerful explosive mines under the German lines. In World War II, the Imperial Japanese Army built dense tunnel complexes on many of the islands it sought to hold against invading U.S. troops. The Japanese found they could double the level of enemy casualties by defending from within tunnels rather than trying to stem the attacks at the beaches. At Iwo Jima, use of tunnel complexes resulted in 27,000 U.S. casualties, including nearly 7,000 killed.

Today, the most elaborate system of tunnels dedicated to war are probably in North Korea. These are deeply buried, probably impervious to even the largest conventional bomb or missile strikes. U.S. and South Korean forces conduct extensive training designed to enter and ultimately destroy such complexes. The North Korean tunnels are much larger, better defended and full of weapons far more advanced than those in Gaza.

The U.S. and NATO allies have their own tunnel complexes. The Cheyenne Mountain Complex in Colorado, run by the U.S. Space Force, is a massive defensive bunker hardened against nuclear strikes at the heart of the North American Aerospace Defense Command. When I was the supreme allied commander of NATO, we had a smaller version of Cheyenne Mountain near my headquarters in Mons, Belgium.

Those facilities are fundamentally different than the tunnel complexes of Hamas, North Korea, Iran and at Tora Bora in Afghanistan, once the hideout of al-Qaida and now controlled by the Taliban. Russia claims it has used tunnel systems to get behind Ukrainian lines for surprise attacks. Complexes operated by rogue states and terrorist groups exist to allow invulnerability to Western intelligence and conventional military strikes.

So, what lessons can war planners learn from the Israeli military’s struggle to win control of Hamas’ underground maze?

First, traditional intelligence systems must put greater emphasis on the tunnel systems of Iran (concealing its nuclear program), North Korea (hiding not only nuclear weapons but also the launchers to deploy them) and terrorist groups. Technical means — mobile-phone monitoring, cyber surveillance — and human intelligence (embedded operatives) have to shed more light on what is happening in the darkness.

U.S. and allied troops will need better preparation for combat operations underground: training for the destruction of tunnel networks of significant size; engineering integration with combat units to include not only explosive means but, for example, flooding tunnels with water; practice with night-vision devices in highly confined spaces; use of special forces as shock troops underground (similar to the “tunnel rats” of Vietnam); and psychological preparedness for subterranean warfare so different from operations above ground.

Another key lesson of Gaza is that we need to apply new technologies. These include intelligence systems that can detect and measure tunnel complexes from space or using long-dwell drones. (This would potentially include hyperspectral technology — high-resolution imaging based on information across the electromagnetic spectrum — to see the movement of earth as tunnels are expanded.) Also necessary are unmanned above-ground capabilities — sonic, infrared and light-detecting — that can operate ahead of human troops to reduce casualties. It would be useful to find new ways to make life underground unpalatable: reducing air and water for example, or by creating unpleasant vapors.

Obviously, the presence of dozens of Israeli hostages in Gaza makes using all such means far more difficult. So, count on other adversaries to take a page from Hamas’ book and start conflicts by kidnapping a substantial number of civilians or military personnel. Doing all we can to be prepared for “kill and grab” operations is critical.

Tunnel warfare is terrifying, and preparing for more of it will unfortunately be a crucial element of the 21st century battlefield. Submarines first became high-level predators in World War I because of their ability to dive below the waves and surface to gather intelligence and kill. Terrorists and rogue states are increasingly going to be thinking about how to do the same from beneath the surface of the earth.

James Stavridis is a Bloomberg Opinion columnist, a retired U.S. Navy admiral, former supreme allied commander of NATO, and dean emeritus of the Fletcher School of Law and Diplomacy at Tufts University. Stavridis is also vice chairman of global affairs at the Carlyle Group. He is on the boards of Fortinet and Ankura Consulting Group, and has advised Shield Capital, a firm that invests in the cybersecurity sector. This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.