What's driving the bitcoin surge? Experts weigh in.

Bitcoin is on a tear.

The largest cryptocurrency has soared more than 20% over the last four days, approaching an all-time high. By contrast, the S&P 500 has dropped slightly over that period.

On Wednesday, the price of bitcoin surpassed $64,000 for the first time since November 2021.

The upsurge stems primarily from an explosion of investment in bitcoin ETFs, a novel investment vehicle approved by the Securities and Exchange Commission last month, analysts told ABC News. That initial burst of gains, they added, triggered a stampede of investors fearful of missing out on the returns.

"It's been a wild ride," Katie Stockton, the founder of market research firm Fairlead Strategies, told ABC News.

A bitcoin ETF (exchange-traded fund) allows investors to buy into an asset that tracks the price movement of bitcoin, while avoiding the inconvenience and risk of purchasing the cryptocurrency coin itself.

For instance, an ETF for gold allows individuals and institutions to put money on the price movement of the precious metal, rather than having to buy, transport and store the physical item.

A bitcoin ETF, in turn, gives investors access to the cryptocurrency market without facing the technical impediments and fees associated with navigating a crypto exchange.

Soon after the SEC approved the new investing option, a slew of bitcoin ETFs became available, including offerings from legacy firms such as Fidelity and Franklin Templeton.

The new crypto alternatives unleashed billions of dollars in investment within weeks, Bryan Armour, the director of passive strategies research at financial firm Morningstar, told ABC News. The nine leading bitcoin ETFs have received a combined $10 billion since last month, Armour said.

"There has been a very successful launch for pretty much all of these ETFs," Armour said. "It's just crazy for seven weeks on the market."

When investors place their money in a bitcoin ETF, the funds in turn purchase bitcoin, increasing demand for the cryptocurrency and potentially causing a jump in price, Armour added.

Since bitcoin ETFs gained approval on Jan. 10, the price of bitcoin has skyrocketed 30%.

"There has been significant trading volume," Armour said.

The rally in recent weeks gave rise to an additional wave of investment when traders witnessed the initial price spike and sought to jump on board, said Stockton, of Fairlead Strategies.

Over roughly a week in the middle of February, the price of bitcoin hovered in a "tight range" at about $51,000, Stockton said, adding that when it broke past that threshold on Monday, the new heights stoked optimism and an onrush of investment.

"The run-up that we've seen over the past four days has been really explosive," Stockton said.

Despite the breakneck pace of gains in recent weeks, some analysts cautioned about the past volatility of bitcoin and the possibility of an imminent price plateau, or even downswing.

In the immediate aftermath of the bitcoin ETF approval, for instance, the price of bitcoin dropped 15% before rebounding, Armour said. Over the past five years, he added, bitcoin has plummeted more than 40% on four separate occasions.

"Investors could expect it to either go up substantially or drop in half," Armour said. "Anything can happen."

James Butterfill, head of research at digital asset management firm CoinShares, acknowledged concern about bitcoin's rapid price ascent, but also pointed to reasons for optimism.

"When you see the price move so dramatically higher, it always worries you a little bit," Butterfill told ABC News. "Is it sustainable?"

However, Butterfill notes that the price surge has coincided with a period of stubbornly high interest rates, suggesting that the jump in demand owes little to excess cash in search of a place to land.

"It's not some crazy speculation," Butterfill said. "There's genuine demand for this."