Bitcoin miners passed a significant milestone over the weekend, when they mined the 16.8 millionth bitcoin from the cryptocurrency’s planned total of 21 million coins.

This means that 80 percent of all bitcoins that will be in existence have already been mined. According to estimates, bitcoin will reach its final coin figure sometime in 2140. (See also: How Does Bitcoin Mining Work?)

Over the years, bitcoin has adjusted the number of coins in circulation through a complex calibration of miner rewards and problem difficulty. Bitcoins are awarded to miners who solve complex mathematical problems through intensive computation. The reward number is halved every 210,000 blocks, per bitcoin’s original algorithm.

Immediately after bitcoin’s launch, miners earned 50 coins as reward for solving problems. It was cut to 25 in 2012 and 12.5 in 2016. In two years, miners can expect 6.25 bitcoins as rewards. The difficulty of problems has kept pace with rewards. As the number of rewards has decreased, bitcoin’s problem difficulty has increased, thereby making it more difficult and computation intensive to earn the coin.

Read more: Investopedia