A photo illustration of the digital Cryptocurrency, Litecoin (LTC), Bitcoin (BTC), Ethereum (ETH) and Ripple (XRP) are seen on September 13 2018 in Hong Kong, Hong Kong.

Amid a sell-off in the cryptocurrency markets that began Wednesday, Bitcoin's market capitalization fell below the $100 billion mark on Thursday — a level not seen since October 2017.

As of 3:26 p.m. HK/SIN (2:26 a.m. ET Thursday), the market cap of Bitcoin stood at $98,194,458,586, according to Coinmarketcap.com.

That move came after a major sell-off in the cryptocurrency markets on Wednesday, which continued into Thursday in Asia.

BKCM founder and CEO Brian Kelly told CNBC's "Fast Money" on Wednesday that the sharp downturn had to do with bitcoin cash, which splintered off in August 2017 from regular bitcoin with the goal of being able to process more transactions.

Bitcoin cash is doing a "hard fork" or "effectively a software upgrade," the cryptocurrency fund manager said. "When you do a software upgrade, everybody usually agrees. But in this particular case, everybody is not agreeing."

As of 3:29 p.m. HK/SIN (2:29 a.m. ET Thursday), Bitcoin stood at $5,564.70, according to Coindesk. Bitcoin had been trading comfortably around the $6,400 range for the majority of the fall, in stark contrast to the volatility it experienced the rest of the year.

Watch: Why Gary Shilling is short bitcoin