Thanks all for the answers!

It is definitely a significant amount of money, so I am really concerned about doing things right from the beginning! I don't think I want to declare it as "other income", not to run into the risk of raising suspects about it being dirty money (this is also true when moving a large sum from an exchange to my local bank account I guess). And I am totally OK with paying taxes on the whole sum ($0 cost basis), as it won't significantly affect taxes in my case.

I would assume that not all tax advisors are equally qualified on this specific topic: are there any sufficiently experienced that somebody can suggest?

I am currently in the US, so if I sell in the near future I'll definitely have to deal with IRS. Does anybody know what would I need to submit to IRS in this case?

At the time I got the coins I was a resident of Italy (the value at the time was definitely below 1k EUR).

I have a bunch of bitcoins, result of mining and buying from many years ago. Now my problem is that I have not kept track of transactions and cost basis of those days. And I have also changed wallets a few times in the meantime: all I know is that the most recent transactions are dated 2018 when I moved all my bitcoins to my current hardware wallet.

My question is simple: how am I supposed to pay taxes if I sell my coins? Is it admissible to declare a cost basis of 0 on my tax declaration, and pay capital gain taxes on the full sale amount? I don't see it being a problem, as it would basically correspond to paying the most possible taxes on that amount, but I am not sure if I am required to attach documentation when filing taxes, for instance the cost basis or proof that I held them long enough to qualify for long term capital gains. Any suggestions for good sources of information on this? Good tax advisors for bitcoin?

Thanks!