Are you serious? Burying something underground is the most basic instinct of every living being. Everyone, especially old people does that, they bury jewellery and money in the house or in the yard. This method is safe if you bury it deep under the ground where metal detectors won't be able to detect it but you have to bury it in a way that it shouldn't be noticeable for anyone that this part of the ground has been dug before.
Is it a safe method? That's very individual but I think there are way better other methods. Instead of burying money, buy a Bitcoin, get a hardware wallet, send coins to this hardware wallet and keep this wallet somewhere safe. Your buried money will become worthless over time because of inflation but your buried Bitcoins will become much more valuable because of many reasons which I am not gonna list here, that would take too long.