From my home AIS receiver, elevation 1000 ft and a few miles from the ocean, I regularly receive AIS signals 1000 NM away, and occasionally as far as 2000 NM. These are solid signals, too, while they last. This is due to "tropospheric ducting", where the discontinuities in the atmospheric layers act as waveguides.
From the boat
, with the antenna
about 40 ft (the upper spreader) 100 NM is the maximum range seen so far.