I have a dormant project (because my sdr died and I need a new one, and after moving house I need to build a proper antenna) that involves listening to maritime MF DSC calls. (8414.5MHz), and plot the call on a map. These DSC calls are mostly distress and similar, so yhey don't happen often, but the TX power of ships that send them gives these calls such a long range that you should be able to pick them up almost anywhere.