I think this story is at least 40 or 50 years old as I mainly read classic SF when I was a teenager. All I can remember about it is some explorers or soldiers were exploring an enormous seemingly endless featureless plain that starts as a gentle slope and then gets steeper and steeper over the days. I can't remember anything about the characters, or how the story ends. I just remember the creepiness of the featureless slope getting steeper and steeper.
Mouth of Hell by David I. Masson
The explorers on the new planet find a deep hole in the planet's surface. It starts as a gentle slope but gets steeper and steeper. They only manage to get a couple of kilometres down before the atmospheric pressure becomes too much to stand.
A few years later a specially build aircraft tries to find the bottom, but cannot quite make it. Radar says the bottom is forty two kilometres below the planet's surface.
Thirty years later the bottom is reached, and ... but that would be a spoiler.
While it is a bit of a long shot, there are some overlapping features with "The Inverted World" by Christopher Priest.
Since you remember so little, perhaps, some of the details you remember are in fact a bit different. So:
I think this story is at least 40 or 50 years old as I mainly read classic SF when I was a teenager.
Published in 1974, and a SF classic.
some explorers or soldiers were exploring
The main protagonist is at some point an explorer, charting the terrain ahead and some time behind the moving city he is from.
an enormous seemingly endless featureless plain that starts as a gentle slope and then gets steeper and steeper over the days.
The terrain is not a featureless plain, however, as the explorer moves further away from the city, he experiences strange physical phenomena, among other things, the terrain is sloping steeper and steeper, until at some point he sees before him (as far as I remember) an endless vertical wall of terrain disappearing topside.
I just remember the creepiness of the featureless slope getting steeper and steeper.
I don't have quotes, but the descriptions deal a lot with the physical weirdness of the world.
I don't want to reveal spoilers, but the world in question appears to be
not a spheroid, but a tractricoid