List mining as either "Accounting Services" or "Database Services". This is on the premise that what mining does is maintain the Bitcoin account books (the block chain).
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Like any business, you can deduct expenses for hardware, software, networking, and utilities.
This is pretty much what I do -- report it as income with a suitably vague heading.
I was at one point tempted to report it under "income from illegal activities" out of an abundance of caution. But I suppose putting anything other than "zero" in that box is pretty much begging for an audit.