Filings submitted by the $9 trillion multinational investment manager BlackRock reveal that the firm has made significant investments in two leading publicly traded Bitcoin (BTC) mining firms.

A June 30 filing submitted to the United States Securities and Exchange Commission, unearthed by Forbes, shows that BlackRock owns a 6.71% stake in Marathon Digital Holdings and 6.61% of Riot Blockchain.

In total, the investments are valued at nearly $384 million, with BlackRock having purchased nearly $207 million worth of Marathon stock and $176 million worth of Riot stock.

According to ETF.com, BlackRock’s iShares Russell 2000 ETF holds more shares in Marathon and Riot than any other exchange-traded fund, while iShares Russell 2000 Value ETF ranks third on the same measure.

The investments are not BlackRock’s first in the digital asset sector, with the firm filing an application with the SEC in January for two of its funds to purchase cash-settled Bitcoin futures contracts before revealing in April that its BlackRock Global Allocation Fund had bought 37 BTC futures contracts from the Chicago Mercantile Exchange.

With few products offering institutional investors regulated exposure to the crypto markets in the United States, Bitcoin mining stocks have become an increasingly popular investment in recent years.

While BTC is up by roughly 288% over the past 12 months, Marathon’s stock has surged 754% and Riot has gained 848%.

Related: You can already invest in hundreds of ETFs with exposure to Bitcoin

Fidelity Group and Vanguard Group are among the large firms obtaining significant exposure to the BTC mining sector in recent months.

Vanguard’s Total Stock Market ETF and Information Technology ETF rank as the fourth- and fifth-largest funds by RIOT holdings, while the firm’s Small-Cap ETF and Small-Cap Growth ETF are the fourth- and fifth-largest ETF holders of MARA shares, according to ETF.com.