Details of the work by amateurs at VLF can be found at the Sub-9kHz Amateur Radio website. Included there are some of my earlier experiments at VLF using earth-mode (through the ground) communications as well as LOTS of data on serious, long distance experiments by radiated signals below 9kHz.

Some of my own earth-mode results are recorded, a bit like a blog, here https://sites.google.com/site/g3xbmqrp/Home/earthmode .