spunker mentioned longwave. That was the first place I went after the MW band. That was my first experience hearing anything other than beacons on LW. I was amused by how, during the day, several of the channels had two clearly audible stations. No beating, no whistles, just two clear bits of audio, one dominant, one in the background. That's pretty neat.
I fell in love with this tuner because of the waterfall graphic. It really is eye-opening to see just how much activity there is between 60 Hz and 30 MHz over in the Netherlands. I heard tons of what sounded like buzz saws, must be a lot of data going out, but for what? Then there were the beacons, the hams, the rtty or whatever it is nowadays. And the mystery stations. A silent AM carrier appears to be a straight line with no variation. But some were more mysterious. Some were wider, with repeating patterns in the spectrograph. One transmission I came across transmitted two bursts every minute or so, and on the graph appeared to be shaped like two commas!
Someone in the comments mentioned Russian radar around 19 MHz but I never heard any of that, or if I did it had moved frequencies. All I heard up there was some sort of two-way CB like communication in FM mode around 27 MHz.
