Buckle up.

Bitcoin this afternoon slid under US$30,000 – a level technical analysts have been saying for weeks was crucial to defend, lest the original cryptocurrency plunge back down to US$20,000 or so.

BTC’s ascent in late 2020 and early 2021 was so meteoric that there precious few resistance levels to serve as support on the way down before then, analysts say.

At 5pm AEST, Bitcoin was trading at US$29,728, down 6.5 per cent in the past 24 hours.

Ethereum was changing hands at US$1,759, down 7.7 per cent.

Most altcoins were suffering double-digit losses, with NEM the worst hit, falling 22 per cent.