My uncle is ill and doesn't have much longer to live. He's been tying up loose ends and the other day he gave me a briefcase full of $20 bills, neatly stacked. If my math is right it's about $100k. I'm pretty sure it's legit, he owns a small shop and distrusts banks and has been pretty successful. Still I can't exactly deposit it in my bank account without drawing suspicion. What is the best way to protect this money? Would depositing it directly in the bank create problems?
It is absolutely legal to have and/or deposit large cash amounts.
There are federal reporting requirements for such transactions over 10k (for the bank, not for you!), and the government agency filters such transactions for potential money laundry. If they contact you, give them the correct story, and it will be fine if it is believable.
It becomes much more difficult if you claim you find such a box of money every week...
One option is that your uncle deposits the money, and then transfers it electronically to you as an official gift (and you can execute this for him as his agent). That way, his name is on the cash transaction, not yours.
Have your uncle fill out the exemption form for gift taxes, take the money to the bank and deposit it at the bank. The bank will file a currency transaction reporting form, but you will have the tax form to show where you got the money in the event the Secret Service or the FBI ever followed up, which they wouldn't unless you also are engaging in criminal activities and they are investigating you anyway.
In that case, you would have the tax form to support the origin regardless. Just be honest if the teller asks you where the money came from. The teller will record your answer.
After you are done with that, go to a reputable broker and have them help you invest the money.