Bitcoin (BTC/USD) has broken through its post-$500 low of $353, tumbling 17% to $325 on Bitstamp in a span of 14 hours.

Bitcoin had held its ground above $350 since last week’s dramatic rise to $500 and subsequent fall. Today’s breach signals a further unwinding from its sharpest gains in 18 months, which appear to have been fueled by speculation out of China, and perhaps even a Russian Ponzi scheme.

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One of the characteristics of the rise was the increasing price premium on Chinese exchanges, which at times grew to 10% over the average price of their peers. Currently, this premium has shrunk to less than 2%, which is not far off the typical spread seen during calm times.

Another clue to the changing climate is the significantly reduced volumes, which hit record levels last week.

Bitcoin is now on the verge of turning negative for 2015. At its peak of $502, bitcoin had suddenly gained an impressive 57% for the year after spending the first 10 months in negative territory.

The drop is bitcoin’s steepest in a 24-hour period since January’s plunge to below $200. The selling was the most intense during the past 2 hours, when it accelerated after breaching the $350 support.

The dramatic rise and fall now resemble last year’s November bubble and further call into question bitcoin’s readiness for the mainstream. While 2015 was by far its most stable year ever, bitcoin has not yet proved itself immune to sharp speculative spikes or manipulation.

Litecoin, which briefly surpassed Ripple in market cap last week, tumbled as well, and briefly poked below $3.00. But its losses were limited to 10%, which is at least consistent with the generally suppressed movements during bitcoin’s rally. The LTC/BTC rate thus rose to 0.0093.