The above image by Diego Delso is of the Uyuni salt flat lake at sunrise on Feb 4, 2016, 5:55 local time. Salt flats (or salt pans) are level surfaces formed by the seasonal evaporation of shallow lakes. They should offer some good opportunities to observe the curve of the earth.
The above image has been offered as evidence of a lack of curvature. It would be interesting to geolocate it, and find out what would be expected here.
The salt flat lakes are around -20.133775, -67.489133 - but note there are several in the vicinity. I've not found the spot yet.