I think you need to look into Airy's analysis of
the diffraction shadow zone.
When radio was
demonstrated late in the 19th century, many physicists
and math dudes were consulted to try and analyze
and thus predict signal levels in the diffraction
zone. The curves given in the ITU-R tomes (I forget
the reference numbers until I get back to base)
for signal level vs distance over a smoothly curved
earth are heavily based on the work of Airy, who
seems to have been the most successful of the many
who looked into the problem.
I believe the mechanism
is a two-component dielectric trapped wave, that
propagates partly in the high dielectric constant
medium (the ground) and partly in the air (k=1)
immediately above it. There has also been some work
done in the last 15 years for the same mechanism
but between sea water and the air. However, as it
relates to clandestine submarine communications,
it is still classified as far as I know.
At bottom, life is all
Sucking in and blowing out.