(CNN) -- I'm looking down from the open door of a Black Hawk helicopter at a straight drop of 2,000 feet. I don't get vertigo, but this is a long way down.
A cargo ship smears a white foam trail behind it, far beneath my feet, and I imagine how long it would take for me to fall to its deck.
But I don't have long to ponder this, as another roar of a Hellfire missile, audible even above the noise of the rotors above me, signals the Apaches are shooting again.
Thankfully their target isn't a North Korean ship or aircraft, but instead a small uninhabited rock 20 miles from the Kunsan Air Base in South Korea. This is how the U.S. military checks its readiness on the Korean peninsula.
Once or twice a year, the 24 Apache helicopter gunships head out to sea and the inert rock. Each rocket costs on average $50,000 and each helicopter is firing seven of these weapons. That's $8.4 million I just watched slam into a cliff.
The men here have vast experience. One tells me about being shot down in Afghanistan two years ago. Here the threat is quieter, more potent and much larger.
The North Koreans have successfully tested a nuclear weapon and their army numbers more than a million men, but these pilots rarely ever see the enemy.
The Apache is a vital part of the American deterrent here. I ask Col. Joe Bassani who commands the Apaches, whether the North Koreans have anything to counter their deadly firepower. He laughs and says nobody has.
But he is not complacent about the threat posed from the North. The sinking of the South Korean warship the Cheonan in March reminded the South Koreans that their pugnacious neighbors were capable of unprovoked aggression.
Forty-six sailors died and there has been a distinct stiffening of resolve in Seoul, where citizens have lived for 57 years in the knowledge thousands of artillery guns are trained on their city.
It's why Bassani thinks it is vital to maintain "the razor's edge," ensuring his men are in a constant state of readiness, with everyone here hoping they will never be called on.