At 6:30 a.m. on July 20, Bitcoin hit $29,600. I don't know if Elon Musk and the financial brass at Tesla regard that price range as a milestone, or even took notice. But to the EV-maker's investors, it represents a landmark—and a cautionary one. At that price, Tesla's realized and paper gains on its famous Bitcoin investment that once reached nearly $1.5 billion, or double what Tesla paid, have vanished.

Musk bought, by Fortune’s estimates, 46,000 Bitcoin between the start of this year and early February at an average price of $32,600. In Q1, Tesla made the smart move of selling 10% of its holdings, or around 4,600 coins, at $59,100, near Bitcoin's all-time peak. That sale brought in $272 million and delivered a pretax profit in Q1 of $101 million.

At the close of Q1, Tesla still held 41,400 coins at a carrying cost of $1.33 billion. By April 14, when Tesla hit its summit of $64,800, those holdings were worth $2.68 billion. Tesla was sitting on paper gains of $1.35 billion on the Bitcoin in its treasury. Add the $101 million profit in Q1, and its realized and paper gains totaled $1.451 billion.

That bounty actually disappeared once before. On June 22, Bitcoin slumped to $28,900, putting Tesla's holdings underwater by $133 million. But Bitcoin staged a comeback, so that in the past month, Tesla's coins were still worth more than it paid for them by a couple of hundred million dollars.

But on July 20, those gains disappeared once again. At $29,600, the coins in Bitcoin's treasury are worth $105 million less than Tesla's purchase price. If Tesla sold them on July 20, it would essentially break even on its original $1.5 billion investment. The brief $1.451 billion windfall that made the Bitcoin wager look so brilliant is gone.

Here's why sounding the alarm when Tesla's holdings fall to breakeven or below is so important. It should alert investors to the damage should Bitcoin's price keep falling. If Bitcoin goes to $25,000, Tesla will suffer a $300 million hit to its earnings. If the price falls to $20,000, the loss rises to $500 million. That's about equal to all of Tesla's pretax income in Q1.

Tesla's Bitcoin investment at first looked like another great move by an adventurous visionary. Now it threatens to wipe out a quarter's earnings, or worse. Musk can dispel worries about the Bitcoin gambit if he announces great Q2 profits on July 26 and in the quarters to come. If not, days like July 20, when Bitcoin's holdings fall into the red, will keep reminding investors of the looming wreckage from Musk's reckless bet.