I'd keep it somewhere safe for like a year and keep a good lookout for anyone missing it. If I don't hear anything then chances are it was dirty money anyway, so their loss my gain. If I do hear of someone missing it and I feel it's legit, I would try to find an anonymous way to give it to them.
Getting authorities involved just seems like it would open up a huge can of worms. You'd be questioned as to why you were there, why you took it etc... there's also a chance that if it was involved in a crime you're now a suspect. It may also be a taxable event to handle that kind of money, that would be enough to go bankrupt if that's the case. So yeah I'd lean towards leaving authorities out of it, but I would want to do the right thing if it's not dirty money.
If after enough time has passed I still don't hear anything then I'd start spending it. Depositing that kind of money in the bank would give huge red flags so I would pretty much use cash to buy almost anything, and then use money in the bank for bills only. Basically I would try to balance it out so it's really not obvious that I'm rich as far as the banking system sees me. Maybe deposit some of it once in a while if I need money in the bank for bills, credit card payments etc, but reasonable amounts like a few hundred. Perhaps around Christmas and my birthday.