It probably was originally to store wood for the fireplace.

Wood or planter. Put it to its intended use and send photos!

It's a typical mid-Century modern type of design. Probably looked pretty avant garde 60 years ago, but maybe less stylish now. If it actually was something else, like wood storage, I think the marble is an improvement.

Well, I actually was able to talk to the son of the people who built the house. It was indeed originally a planter box! Thanks for all the comments!

It’d be a fantastic planter box again if its interior is configured correctly!

Это ящик для сушки дров, здесь написано подобное: как сушить дрова

We removed one in our basement when w moved in. It provided a lot of extra brick that will be put to work elsewhere, but didn't take too long to disassemble. Consider flooring continuity, if you're thinking about removing it!

I’ve got a square metal box (sans lid) set into the top of both sides of my fireplace flue. There is no sign of there ever having been dirt or water in them. (Just filled with dust!) The space above is less than a foot. House built in 1955. Could these also have been intended as planters? If not, any thoughts?