They are just lost foreveer. I've read that nearly 4 million bitcoins have been lost forever. Just as gold bars are lost at sea or $100 bills can burn, bitcoins can disappear from the Internet forever. When all 21 million bitcoins are mined by the year 2040, the actual amount available to trade or spend will be significantly lower.
As we know, only 21 million bitcoins will ever exist. it is tough to tell apart lost bitcoins and those which are just being saved for a rainy day. However, the blockchain includes every transaction ever made using the currency. Whenever miners verify new transactions, what they’re actually doing is adding on new blocks onto the blockchain.
So these unused bitcoins just sit on the blockchain until someone uses their private key to move them to another address. There are ways to analyze the blockchain and see how long bitcoins have been idle or unused or haven’t moved around for a period of time. However, there is no way of knowing whether they are permanently inaccessible or not.
Bitcoins can be lost or misplaced because of various reasons including death, hardware failures or even plain carelessness. A programmer in Florida, Laszlo Hanyecz had supposedly bought two pizzas in 2010 for 10,000 Bitcoins. If he had done the same in 2017, the era of the Bitcoin’s zenith point, that particular pizza would have cost him at least $ 10 million.
