Learn Portuguese (Portugal): Os arqueólogos trabalham na trincheira até desenterrarem o fóssil. - The archaeologists work in the trench until they unearth the fossil.

We use the personal infinitive because the action of unearthing has a specific subject: the archaeologists ("they"). Even though the subject is the same in both halves of the sentence, adding the third-person plural ending -em to the base verb desenterrar makes it crystal clear who is doing the action after the preposition até.

I learned earlier that "until" triggers the subjunctive (até que). Can I use that here instead?

Yes, you could say até que desenterrem using the present subjunctive. However, using the preposition até followed directly by the personal infinitive (até desenterrarem) means exactly the same thing and is highly preferred by native speakers. It is a more concise, natural-sounding shortcut!

The verb desenterrarem looks like it contains the word for "earth". Is that right?

Exactly! It is built around the noun terra (earth or dirt). By adding the prefix des- (meaning "un-" or "out of") and the verb ending -ar, you get desenterrar, which literally translates to "unearth" or "dig up".

What is the plural of fóssil? Does it become fóssis?