[discretion22]

136KHz is available : https://rsgb.services/public/bandplans/#5

and 472KHz (https://rsgb.services/public/bandplans/6/)

and 1.8-2Mhz (https://rsgb.services/public/bandplans/7/)

There a surprising amount of things it can be used for and some really interesting innovation at the bottom end of the spectrum. Well worth getting qualified as an Amateur operator if you want to experiment.