Tharp and Heezen’s mid-1950s “The Floor of the Oceans” map, in hi-res. Just in case Mythos writers needed to research a location for an underwater city or something.
Note especially the curious shape of Greenland. Presumably envisaged that way to show the sub-glacial meltwater lake area, possibly with life, which lies deep beneath the thick ice cap.
In Lovecraft’s day the ocean depths were mysterious unmapped places, plumbed primarily by mariners and those laying undersea telegraph cables. The weirder denizens of the deep were better known, having been hauled up by the likes of the Challenger Expedition and also by occasional startled fishermen. And hunted by the likes of the Arcturus Expedition. The bathysphere (metal diving sphere) was essentially still just an interesting one-off novelty in the mid 1930s. The first modern textbook on oceanography was not published until 1942, after Lovecraft’s death. Only after the Second World War — with war-surplus Navy ships and sonar at the disposal of scientists — could sea-bed mapping be undertaken in detail and over wide areas.