One of the more unusual propagation modes on the VHF bands is Ionosphere Scatter or 'Ionoscatter' for short.
To establish contact, both stations at either end of a link usually run very power into very large antenna systems with a lot of gain and beam at a common point in the D layer of the ionosphere which is about 85 kms above the earth.
It is similar in many ways to troposcatter except the greater height allows a much greater range. With troposcatter, the scatter point in the troposphere may be in the region of several kms above the earth e.g. 9kms for a 500 km path.
For ionoscatter, the radio signal is scattered forward by small changes in the ionization density in the ionosphere at a height of about 100kms or below with a peak at 85 kms. This gives a typical range of about 800 kms to 2000 kms with a peak around 1300 kms.
Ionoscatter works best at the low VHF part of the spectrum (30-50 MHz) although under the right conditions, it can be used at 144 MHz as well.
Now with the advent of new digital modes, the entry level for this mode of propagation is lower and it's possible that many people have already made ionoscatter contacts and were not even aware of it.
At the 2019 RSGB Convention, Richard, G4CDN gave a talk titled "2m Weak Signal Ionosphere Scatter". In it, he gives an outline of his experience of ionoscatter and advocates the use of the QRA64 digital mode.
Additional info...
1) Ionoscatter lecture from 2002 by OZ1RH
