.
The above images of earth contain large areas of the Pacific and Atlantic oceans, much of which are featureless vast abyssal plains.
.
When observed by any of the technological means available (they're too deep under water to be seen by eye), they appear flat in any immediate area of say less than 300 square nautical miles in area, something like the Bonneville salt flats.
.
In order to get an idea of what the earth's curvature looks like on an abyssal plain you have to consider a much larger area.
.
Take the area of the Pacific Ocean bounded by Hawaii on the north, and Tahiti and Easter Island on the south.
That would be a 1-inch diameter circle in the middle of this picture:
.
That would have the area roughly equivalent to the area of the 48 contiguous States and Mexico combined, or 32 million square nautical miles.
.
That means an area 100,000 (one hundred thousand) times larger than the Bonneville salt flats.
.
To view such an area from above ground you would need to be higher up than LEO of the ISS.
.
But nonetheless, the abyssal plain it covers has no appreciable features to speak of.
If you were on the ocean floor anywhere inside this area it would appear "flat" in all directions.
But when viewed from above or anywhere outside the earth's curvature would be evident.
Except for the fact that the water of the ocean would be in the way so you wouldn't be able to see the abyssal plain directly.
.