100 years ago there was practically no microwave radiation on Earth. There is no natural substance that emits microwave radiation at the level of even a single microwave oven. Today, microwaves are literally everywhere, from your kitchen to your cell tower to your office to your airport. They are an essential part of life.
The term “microwave” refers to any broadcast on a frequency between 300MHz and 300GHz, although in practice it generally means frequencies above the broadcast TV bands. Your router works on microwaves and so does your cell phone, but where microwaves really get down and dirty is up in the bands above 6GHz. That’s where you’ll find satellite broadcasting, long-distance lines, and other point-to-point communications. Microwave communication was first used to replace long copper lines for phone service and that’s still a big part of what it’s used for, especially with communications between cell towers and land lines.
Microwaves are everywhere, they’re harmless, but they’re all man-made. If we stopped broadcasting on microwave bands, the earth would instantly be free of them once again.