The wild ride for Plug Power investors continued on Tuesday.

Shares of the hydrogen technology provider plunged 21.5% in after-hours trading to $1.68 after the company announced a capital raise.

Tuesday evening, Plug said it would offer $375 million of convertible notes, which can become more shares outstanding. That dilutes existing shareholders’ stake in a company.

Plug says it will use about $243 million of the proceeds to repay principal and interest on its 15% secured debentures. That is expensive funding.

The company, however, doesn’t generate earnings or positive cash flow yet. It has been using about $100 million or $200 million per quarter to build its business, which hopes to bring low-cost, commercially viable hydrogen technology to the U.S. economy.

Plug ended the third quarter with about $550 million in cash on its balance sheet and about $1.1 billion in debt, according to FactSet.

The market value of Plug Power’s stock ended Tuesday at about $2.9 billion, according to FactSet.

Shares gained 2.6% in regular trading, while the S&P 500 and Dow Jones Industrial Average lost 0.8% and 1.1%, respectively.

The stock has traded as high as $4.58 over the past 12 months, reaching its 52-week high in October, and as low as 69 cents. The almost $4 spread is roughly 200% of the after-hours stock price.

That’s a lot of volatility. That calculation for, say, diesel engine maker Cummins yields about 50%.

All that volatility, surprisingly, left Plug stock flat year to date, through Tuesday’s trading.