It's fair to say that some monoliths could count as monuments, but it's stretching it to say that it is a synonym.
Block, column, monument, pillar and statue are all synonyms for the word monolith.
monolith
A monolith refers to a large, single, and typically impressive structure or monument. In the context of technology, a monolith can also describe a software architecture where the entire application is structured as a single unit.
The root word "litho" in "monolith" originates from the Greek word "lithos," which means "stone." Therefore, "monolith" literally means "single stone," reflecting the idea of a large, single upright stone monument or structure.
an obelisk, column, large statue, etc., formed of a single block of stone
Mt Augustus (Burringurrah, as it is known by Wajarri Yamatji aboriginals. It is significantly larger than Uburru, also in Australia. This monolith is also called Ayers Rock by the white population.
The verb is eulogize.
Living Monolith was created in 1969.
Monolith Soft's population is 2,012.
The population of Monolith Productions is 100.
Pokotia Monolith was created in 2001.
Oh, dude, an antonym for obelisk? That's like asking for the opposite of a tall, skinny monument. I guess you could say a ditch or a hole, because they're like the opposite of tall and pointy. But hey, who really thinks about antonyms for obelisks anyway?