Okay kiddo, have you ever played the game Minesweeper on the computer? Well, submarine mines in the United States harbor defense work kind of like that, but on a much bigger scale.
A submarine mine is a big explosive device that can be planted underwater. If a ship or boat hits the mine, it goes BOOM and stops them in their tracks.
The United States set up submarine mines around some of its harbors during times of war to protect against enemy ships. Think of it like setting up a big fence around your house to keep out bad guys.
But, they don't just plant these mines randomly. They have to study the harbor and figure out where the enemy ships are most likely to come in from. Then they place the mines in those areas so that they are ready to explode if an enemy ship comes too close.
The mines are usually controlled by people on land who can set them off at the right time. It's like having a remote control to set off a firework display, but instead of pretty colors in the sky, it's an explosion in the water.
Overall, submarine mines are a way to deter enemy ships from attacking our harbors and keeps us safe. But, we hope we never have to use them and that we live in a world where we can all get along peacefully.