If you’re talking about a plural noun, talk about them in the plural: sinkholes are […] portals.
Compound sentences require commas, dammit. (See this face? This is my exasperated-proofreader face.)
Please note that sinkhole is one word.
Sinkholes are now known to be portals to another time and place. All sinkhole locations are now catalogued, but no one has ever returned from one. You volunteer to go next.
I wonder, though, how anyone knows where those sinkhole portals lead, since no one has ever gone into such a portal and returned to tell what’s on the other side. (Sinkhole? No, plot hole!)