If I wanted to talk about treasure owned by multiple explorers, how would I write it?
The explorers' treasure.
The explorer's treasure.
Is the plural form of explorer considered irregular?
In the case of explorer (as with any regular noun; explorer is not irregular), plural adds -s (i.e. explorers), possessive adds -'s (explorer's), but both plural and possessive together add -s' (explorers'). Therefore, your first example is correct: