According to the passage, what is thought to be sunk beneath the ocean?

Directions: Read the following passage and answer the question that follows.

It may be objected to this description, that the Egypt which it presents to the reader is not the Egypt of the maps. Undoubtedly, it is not. The maps give the name of Egypt to a broad rectangular space, which they mark out in the northeastern corner of Africa, bounded on two sides by the Mediterranean and the Red Sea, and on the two others by two imaginary lines, which the map-makers kindly draw for us across the sands of the desert. But "this Egypt", as has been well observed, is a fiction of the geographers, as untrue to fact as the island Atlantis of Greek legend, or the Lyonnesse of medieval romance, both sunk beneath the ocean to explain their disappearance. The true Egypt of the old monuments, of the Hebrews, of the Greeks and Romans, of the Arabs, and of its own people in this day, is a mere fraction of this vast area of the maps, nothing more than the valley and plain watered by the Nile, for nearly seven hundred miles by the river's course from the Mediterranean southwards.