The Terra Luna token fell from a high of $118 (£96), last month, to $0.09 on Thursday.
The collapse had a knock-on effect on a linked token, TerraUSD, which is normally stable.
And spooked investors are now pulling out of major crypto-currencies, sending markets plummeting.
The companies behind stablecoins try to ensure they remain in parity with assets such as the US dollar - with one token equalling $1, for example.
But on Thursday TerraUSD fell to $0.4 according to the trading website Coin Market Cap.
Tether, the most popular stablecoin, also fell off its US dollar peg - to an all-time low of $0.95.
The term "cryptocrash" has been trending on Twitter and Google Search.
And the combined market value of all crypto-currencies is now reportedly $1.12trillion, about a third of its November value, with more than 35% of that loss coming this week.
One Bitcoin is now worth about $27,000, its lowest value since December 2020 and down from a high of nearly $70,000 late last year.
Ethereum, the second largest coin by value, has lost 20% of its value in 24 hours.