What were once glorious forts are now nothing but piles of rubble.
Which is the correct phrase to use here? Is it "piles of rubble" or "piles of rubbles"? Pile of rubble is perhaps wrong as it's forts about which we are talking but do I need to make rubble plural too or is it fine to use the singular form? The doubt arose when I read the plural form being used here:
Most parts of the ruin are just piles of rubbles - Picture of Lelu Island, Kosrae