20% of all Bitcoin mined to date has been misplaced, according to the Wall Street Journal, and it's improbable it will ever be returned to circulation. In raw numbers, that's 3.6 million Bitcoin lost out of the 18 million in the wild, worth around $145 billion at the current market value. That's $29 billion in Bitcoin unaccounted for.
Since Bitcoin is decentralized, there's no mediator that can be called upon to recover a balance when it's lost, with the most common cause being a Cold Wallet being lost or a misplaced Private Key for a Hot Wallet. When this happens, if there's no user-made backup of the Bitcoin Wallet, the Bitcoin is deemed unrecoverable and is therefore lost.
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Interestingly, some people believe that around 60% of all lost Bitcoin is not actually unaccounted for, but rather stored by Bitcoin inventor Satoshi Nakamoto, who last accessed them when he launched Bitcoin in 2009. This is a fair assumption to make, but without solid evidence, there's little substance to support the claim, say many critics.
The fact of the matter is, we don't know what's happened to the 3.6 million Bitcoin deemed to be lost — all we know is it isn't been used. Right now, at least. There's a possibility that some of it could make a return at some point in the future, when people find a reason to tap into it (if the value reaches six figures, for example).
But it's safe to assume that a significant amount is lost, never to be seen again.
This article is an installment in our brand new Bitcoin 101 series — content dedicated to answering all of your most basic Bitcoin-related questions and queries, including where to spend Bitcoin, how to mine Bitcoin, how to spend Bitcoin using a Visa debit card, and the safest and easiest way to convert Bitcoin to USD.