Learn Polish: Ten znany archeolog zaczyna wykopaliska. - This famous archaeologist is starting the excavations.

The plural is archeolodzy. Just like the -anin nouns you are studying (which drop the -in), many other masculine personal nouns have special changes in the nominative plural. For professions ending in -g, the consonant changes to -dz, making the plural ending -dzy (similarly, biolog becomes biolodzy).

What case is wykopaliska in this sentence?

It is in the accusative case because it is the direct object of the verb zaczyna (he is starting). The base form is the neuter singular noun wykopalisko, but excavations are almost always discussed in the plural. For neuter plurals, the accusative case looks exactly the same as the nominative case (wykopaliska).

Why does the sentence use zaczyna instead of zacznie?

Zaczyna is the present tense of the imperfective verb zaczynać (to be starting). If we used the perfective form zacznie (from the verb zacząć), it would change the meaning to the future tense: "This famous archaeologist will start the excavations."

Is the word wykopaliska related to the verb kopać (to dig)?