For reference, assume we'll be working with
this map of the Vile Vortices, but stretching some (or all) of them into lozenges, generally by fitting another triangle at the appropriate spot to what is a triangle in the picture. If necessary, the lozenges could also be turned around a bit in the process, to ensure that they cover the right areas and avoid what I want avoided. Currently, I have some views on that for the Caribbean 'Bermuda Triangle' and some vague ideas about the two African Vile Vortices and I might end up having preferences about the Vile Vortex in Pakistan and environs.
Also note that that there are a lot of hot spots that
aren't part of the Vile Vortices, but rather seem to be confluences of arcane tides and ley lines, places of mystical correspondence or historical occult areas. These will tend to be much smaller and not endure as long, but might actually be just as powerful in the short term.
There are several areas in the world that, while not Vile Vortices, have been comparatively speaking, higher mana throughout all of the time modern occultists have been aware of the supernatural. These include much of the old 'Cradle of Civilization'; areas of the Middle East like much of Egypt, everywhere around the 'Holy Land' of Israel, Lebanon and parts of Jordan and Palestine, most of Iraq and parts of Syria, Iran and Anatolia.
There are small geographic areas with extremely powerful Places of Power in all of these locations and the average mana levels outside the major cities probably approaches Low Mana (-7 to -5 or so), as Vile Vortices can do. In fact, many occultists reject Ivan T. Sanderson's notion of the Vile Vortices entirely or, at the very least, propose adding an additional Vile Vortex to the schema, as the Eastern Mediterranean and Middle Eeast is by many yardsticks every bit as magically active as some of the Vile Vortices.
The geographic area where 'Classical Civilization' was concentrated around the Mediterranean also includes more Places of Power than is average, with Greece and Macedonia being fairly riddled with the places, as well as many islands in the Mediterranean having higher mana than most tourist locations. In general, Mediterranean coasts are higher mana than other places outside Vile Vortices. The higher mana is usually only a level or two above 'normal' (normal being Very Low Mana, ca -8 to -10) and dangerous tears in reality that let in floods of weirdness are not as big a threat as within the Vile Vortices.
London is also a place where there are many Places of Power and the mana levels are definitely higher than in most other such rich and technologically advanced urban areas. According to sources with the '
Shadow Court of HM Queen Elizabeth II', the Very British Conspiracy of occultists, scholars and shockingly lethal public school chaps sporting regimental ties, any preternatural model that fails to account for the centrality of London to the occult world absolutely cannot be relied upon.
Similar 'quasi-Vortices' might exist in other locations, notably China and possibly India, but the PCs' Patron, J.R. Kessler, does not have good contacts with anyone well informed on the status of the supernatural there. In particular, the Chinese government very effectively covers up and denies any evidence of supernatural occurrences on the Chinese mainland.
In general, local mana levels, as well as the concentration of ley lines and Places of Power, correlate pretty well with how much mythology and legendry there is associated with the area. Rural areas tend to be higher mana than urban areas and mana levels rise with sunset, but there are absolutely places, even within crowded modern metropolises, where something uncanny can exist even in the brightest sunlight.
It's unclear whether there's a causal link, but the more secular, materialistic, modern and 'Western' a given geographic area is, the less mana it will tend to have. Buildings of modern steel, glass and high-tech materials, built using designs and methods developed at late TL8, will tend to interfere more with magic than buildings of older designs and materials, but determining the relationship is so far not an exact science. Science laboratories are not usually good for mana levels and any modern building filled with late TL8 technology will be tend to be either No Mana or Very Low Mana (often -10), but there exist documented instances of powerful magical hot spots coming into existence in places otherwise full of advanced technology and high-tech materials.
What does seem clear is that it's not concentrations of people that do not coexist with mana, it's concentrations of people
who don't believe in the supernatural and whose lives are concerned mostly with material things. Certainly, metropolises in countries where belief in the supernatural is strong and avoiding the Evil Eye and the notice of witches is a current concern with most of the inhabitants, can have comparatively high mana just fine. Even in ultra-modern New York, the mana levels in certain neighborhoods seem high enough for all sorts of shenanigans, perhaps because so many who live there believe that the supernatural is a living force in their lives.