Question: Why did it take Starfleet 100 years to find the remains of the Borg and their sphere in the Arctic?
Question: Is the captain's dog still alive?
Answer: Yes. Porthos survives this episode, although many a viewers' sanity and the show's ratings were not so lucky.
Chosen answer: That's just how long it took. We are still digging up artifacts that are thousands of years old. It takes as long as it takes. Add to that the fact that, at the time the Borg came, Earth was still recovering from a massive war, and it's safe to assume that it took a while for complete exploration and research missions to get going full bore again. Also, they didn't know to look there. There would be no reason to simply head off to some random site in the Arctic and start digging.
Garlonuss ★