PUB. 200 (PLANNING GUIDE SAILING DIRECTIONS & ENROUTE) ANTARCTICA 2005 NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY)

"There is almost no dust or solid particles in the Antarctic air and the prevailing winds blowing off the continent have small moisture content. Consequently, the visibility is usually very good and often exceptional, a fact which, if not appreciated, may lead the observer into serious error when judging distances. An object which may be thought to be but 5 miles distant could well be 30 miles away, and mountains have been sighted at 300 miles"

The tallest mountain on Earth is just under 30k ft tall, at a 300 mile distance the drop due to curvature is over 50k ft. There isn't a single reason a mountain can be sighted from 300 miles away on a globe with the radius of 3959 miles and clear conditions

https://dizzib.github.io/earth/curve-calc/?d0=300&h0=300&unit=imperial

https://www.vidlii.com/watch?v=0eYATaHjuvU
https://www.vidlii.com/watch?v=bQObVcEHXNW
https://wiki.247flatearth.net/index.php/Observations