My WIRS tales claim seven (7)  parallel universes, of which four of the 'other earths' even have very similar day-lengths to ours.

One featured what we'd call 'Atlanteans', with 'Third Law' tech. Snag is they over-fracked their failing geo-thermal tap, covered equivalent of our Eifel region in flood-basalt, with volcanism still active. Local survivors fled via 'World Gates' with portable tech, cut a swathe through our 'High Bronze Age'. Seems Hancock {Spit !!} was half-right about his 'Clever Ancients'. But, when those 'magic' widgets ran short of 'manna', their owners managed to similarly 'Chernobyl' another geo-thermal tap. Blew Thera (Santorini) half-way across Eastern Med....

Most 'Atlanteans' who'd simply 'hunkered down' became increasingly isolated, in-bred, toxic, degenerate. Some used the few remaining 'World Gates' to access, even hunt other parallels. Those who came here likely spawned tales of devils, Weres, Vampires and 'Dark Fae'.

Atlantis' Westerly fringe, cleansed by Atlantic storms, may have kept a modicum of civilisation until our Middle Ages. Hence, 'Light Fae' and such...

The anaerobic 'Seventh World' is weird, possibly a mini-Neptune. Its bigger denizens may have spawned the Cthulhu Mythos. Fortunately, no Gates are big enough to pass the 'apex' beasties, they can only feel around with kraken-scale tentacles. Lesser types tend to be protean shape-shifters, collapsing to green Ectoplasm when stomped...

Between these, the post-post-industrial (??) Lizard People and a zoo of 'False Positives', my 'Walsingham Institute for the Regulation of Supernaturals' is kept busy...