A crater is a typically bowl-shaped depression in the ground caused by a sudden, violent release of energy. And the result of that sudden energetic event is often the loss of lives in the immediate area. More rarely, a crater saves a life. One such crater, in Quang Tri Province, Vietnam, US Army Corps of Engineers grid coordinates 317448, saved five young American soldiers. It also helped advance my understanding and appreciation of physics.

The first time I saw a real crater any closer than the surface of the moon was on Monday morning, May 2, 1966. I was a 17-year-old paratrooper making my sixth jump at the US Army Quartermaster School, Parachute Packing and Airdrop Course, Fort Lee, Virginia. My chute popped open as an M37 truck, weighing about three tons, flew out of the rear of a C-130 at the far end of the DZ. Because of heavy oscillations, the two parachute releases fired prematurely, dropping the truck while it was still several hundred feet in the air. It plunged, engine first, and cratered into the earth, vanishing under a cloud of dust.

I landed and ran to the truck. From my studies of firearms ballistics, I knew that (on Earth and disregarding wind resistance and sectional density) a falling object increases its speed by 32.2 feet per second for every second it falls, and that as velocity doubles, energy quadruples. The truck hit the earth at about 250 miles per hour, or 365 feet per second. That’s about half the velocity of a typical 230-grain .45-caliber bullet. But this was a three-ton-truck! Since a .45 bullet weighs about a half ounce, and an M37 truck weighs conservatively three tons with load, or 96,000 ounces, the truck struck the earth with the energy of nearly fifty thousand .45 bullets. (We mustn’t forget the inverse effect of energy—the truck was traveling half the velocity of a .45 round, thus, by unit of mass, releasing one-quarter the energy.) The destroyed truck and the crater formed by that impact was the result of kinetic energy—that is, mass in motion.

That afternoon, I saw my second crater and observed the 102-year-old effect of a chemical reaction. I was with my parents, who were visiting before I shipped off to Germany. Driving outside the city of Petersburg, Virginia, we came to the Civil War site commemorating the “Battle of the Crater.” Before us was a cavernous hole in the ground 170 feet long, 110 feet wide, and 30 feet deep, covered with grass. In the early morning hours of Saturday, July 30, 1864, Union troops detonated eight thousand pounds of black powder beneath Confederate trenches during the Siege of Petersburg. The explosion blew a hole in the Confederate lines and immediately killed 278 men as thousands of Union troops charged forward into the darkness. But instead of maneuvering to the flanks, they ran into the crater, seeking shelter from rifle fire, and became bogged in loose, pulverized earth. The Confederates counterattacked, and by dawn, they had encircled the Federals with muskets and cannons, killing 504 men and wounding 2,000 more in a horrific “turkey shoot.”

The igniting black powder used to blow the hole in the lines, and to fire all the rifles and cannon on both sides, was chemical energy. This happens when the bonds of electrons are broken at the molecular level, where more than one atom is held together by sharing electrons. The result is an exothermic oxidation-reduction reaction, in which heat is given off. If the transformation happens slowly, it can appear nonviolent, as, for example, the creation of ferric oxide (rust); but if the chemical change happens suddenly, it will appear as an explosion. On the other hand, when a solid like black powder is suddenly turned to gas in a millionth of a second, it needs lots of space, and everything is flung out of the way, whether it’s tons of dirt, body parts from hundreds of soldiers, or the minie rifle ball from a Civil War musket.

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My next acquaintance with craters took place when I was a parachute rigger in Germany and sent to France to retrieve a load of G-11 cargo parachutes. President Charles de Gaulle had demanded that all U.S. troops get out of the country, and I was in a five-ton truck with trailer, driven by a soldier from another unit, when our truck broke in Verdun. Nearly fifty years to the day earlier, in 1916, one of the most horrific battles in human history came to a close in that village.

The next morning, Tuesday, November 22, 1966, was warm and sunny as I walked from US Army Post Verdun to the battlefield. Unlike in the US Civil War, where the only explosives were black powder and nitroglycerine (a high explosive that is also very unstable and, hence, rarely used), science had advanced. Troops were now forced to deal with aircraft and aerial bombs, machine guns, tanks, poisonous gas, flamethrowers, and rapid-firing artillery with high-explosive shells.

In the fierce ten-month battle, modern weapons and ancient tactics had killed 162,000 French and 143,000 German soldiers along just six miles of front lines—the highest density of casualties per square yard in the First World War. On the gently sloping fields, long since covered with thick, lush grass and wildflowers, I saw hundreds of craters filled with water, and rusty metal stakes in the ground, festooned with strands of barbed wire.

Although we experience the detonations of the various chemical agents in battlefield ordnance as explosions, the explosives are actually burning. Low explosives, such as gasoline, black powder, or smokeless gunpowder, have a burn rate of less than five thousand feet a second, whereas high explosives have a burn rate above that. For instance, TNT has a burn rate of nineteen thousand feet—nearly four miles—per second. Because TNT is very stable and has a low melting temperature, it can easily be poured into artillery shells. This was the source of the chemical energy that shattered Verdun’s landscape and the bodies of hundreds of thousands of men.

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My next encounter with craters wouldn’t be clad in a glory of wildflowers. It was near the besieged US Marine combat base at Khe Sanh. In the peaceful countryside of Verdun, I had thought, How did war ever exist here? But in Vietnam, after seeing so many destroyed villages and rice paddies pockmarked with bomb and artillery craters, I thought, How did peace ever exist here?

Sunday evening, April 7, 1968. En route to our Khe Sanh patrol. Sgt. Doug Parkinson center, Bruce Cain right, Bob Whitten far right (KIA 5/8/68), and me left

The Khe Sanh combat base was manned by the Twenty-sixth Marine Regiment, South Vietnam’s Thirty-seventh Ranger Battalion, and a small detachment of US Army Special Forces. It was surrounded by a network of enemy trenches from North Vietnam’s two most elite divisions, the 304th and 325th. Outnumbered and pounded daily by rockets and mortars and by long-range artillery firing from high mountain caves in Laos, the North hoped to create another Dien Bien Phu, where the French suffered a humiliating defeat and lost their war against the Viet Minh. But the French didn’t have the US Air Force and an airmobile division to unleash.

B-52s flying from Thailand, Guam, and Okinawa made continual bombing runs on the encroaching enemy, and fighter-bombers made repeated strikes. On April 1, the marines launched an armor counterattack out of Ca Lu along Route 9 as all three brigades from the First Air Cavalry Division—twenty thousand men and 450 helicopters—leapfrogged forward, commencing Operation Pegasus, the relief of the combat base.

As that massive battle raged, I was on a six-man reconnaissance patrol on Dong Tri Mountain, an eight-mile-wide, 3,300-foot mountain overlooking Route 9 and the combat base. Hidden deep beneath its triple-canopy jungle, our team climbed from bottom to top in search of the enemy. We didn’t find any, but we could hear constant thunder and feel the shaking earth as our artillery fired a hundred thousand shells and as the B-52s dropped seventy-five thousand tons of bombs. The explosive energy released by the B-52s alone was equivalent to five Hiroshima atomic bombs.

Khe Sanh is the most heavily bombed area on earth. But only on our insertion and McGuire-rig extraction did we see the big picture of the seventy-seven-day siege: miles and miles of craters that resembled a moonscape. But they weren’t chalky white; they were red clay. The rich, fertile soil that once gave life to coffee and rubber plantations was now soaked in the blood of 205 dead marines, 59 cavalrymen, and 10,000 North Vietnamese.

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I happened upon my next crater quite by chance on Tuesday evening, August 26, 2014. My wife, Cathy, and I were driving through northern Arizona when I saw a sign for “Meteor Crater.” I had read about the crater and remembered Carl Sagan talking about it in Cosmos. I doubted we would ever be in that remote stretch of Arizona desert again, so we took a detour.

Meteor Crater and me. This is what a twenty megaton bomb could do — Meteor Crater and me. This is what a twenty megaton bomb could do

When we arrived, we saw a massive crater more than 4,000 feet across and 550 feet deep—equivalent to the height of a sixty-story building. Along the entire rim were large boulders, some the size of a small house. Fifty thousand years ago, a huge iron-nickel meteorite, some 150 feet across and weighing several hundred thousand tons, hurtled earthward at 26,000 miles per hour. It passed through our atmosphere with almost no loss of velocity or mass and, in a blinding flash, struck the rocky plain with a kinetic force greater than twenty million tons of TNT. The impact instantly vaporized rock and metal and ejected over 175 million tons of stone, forming a continuous blanket of debris that surrounded the crater for more than a mile.

The atomic bombs dropped on Hiroshima and Nagasaki released the explosive power of fifteen and eighteen kilotons of TNT respectively. Combined, that is about one six-hundredth the energy that formed Meteor Crater. But on October 30, 1961, the Soviets detonated the fifty-megaton “Tsar bomba” in the atmosphere—the most powerful manmade explosion in history. If the Tsar bomb had been detonated below ground, its energy would have made two and a half Meteor Craters. Addressing the dilemma of technology after detonating the first atomic bomb, J. Robert Oppenheimer, scientific director of the Manhattan Project, quoted Hindu scripture from The Bhagavad Gita: “Now I am become Death, the destroyer of worlds.”

When the nucleus of an atom (the protons and neutrons) is broken, it releases a million times more energy than the same mass of a chemical explosive, in which just the outer “shell” of an atom—the electrons—is disturbed. The mighty Tsar bomb seemed to defy nature, but we only have to look back in time on Earth to realize that nature can always outdo man. Sixty-six million years ago, a six-mile chunk of rock struck the Yucatan Peninsula with a force of one hundred teratons of TNT (two million times the energy of the Tsar bomb). The impact formed the Chicxulub Crater, 110 miles across, and hurled so much debris and dust into the atmosphere that the planet cooled for a decade. The resulting mass extinction killed more than half the Earth’s plant life and all the dinosaurs, setting the stage for the eventual birth of ancestral humans sixty-two million years later. Thus, we all owe our existence to the chance collision of that rock with our planet, but I owe a personal debt to one particular crater.

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My front scout, Tony Griffith (KIA 2/5/69), keeping watch by a shell crater — My front scout, Tony Griffith (KIA 2/5/69), keeping watch by a shell crater

It was a cold, drizzly Wednesday morning, September 4, 1968, the last day of my twenty-second and final patrol. My five-man reconnaissance team sat exhausted, awaiting an extraction inside the tattered, dripping vegetation that surrounded a bomb crater. The crater provided a clearing where a helicopter could land. Otherwise, we would have to be hauled out on the 120-foot ropes of a McGuire rig. Since I had done that before with near-fatal results, the crater looked like our safest option. Then, for the second time in five days, we were abruptly attacked by one of our own aircraft.

In the first incident, a Cobra gunship had fired four rockets at us. This time, we heard a helicopter flying low, directly at us. We thought it was an advance bird from our extraction force. It wasn’t. It was a Huey gunship, which suddenly opened fire at suspected enemy positions with its nose-mounted 7.62mm minigun. Then it shifted its fire to us. We plunged into the crater as hundreds of bullets with interspersed tracers sailed overhead, cracking the sound barrier. It sounded as if the muzzles of that many rifles were firing just above our ears. Surrounded by the berm of earth, we tried frantically to melt into the crater while radioing the helicopter to cease fire. The chopper had gold crossed sabers painted on the nose, indicating that it was from the First Squadron, Ninth Cavalry—our division’s most elite aviation force. The gunship instantly quit firing and radioed, “Sorry about that. We were reconning by fire, over.” It flew off, and an eerie silence returned as we groped our way out of the crater and were extracted moments later.

My RTO, Bill Ward, and me after the last attack

Physics is pure, cold logic that explains the structure and behavior of matter. Nuclear reactions have killed hundreds of thousands of people, and chemical reactions have killed millions more. But for the chemical energy that formed the bomb crater at Quang Tri Province grid coordinates 317448, five battle-hardened yet terrified young American soldiers would certainly have died of similar chemical reactions on a much smaller scale.