When the dot-com bubble came apart more than 15 years ago, it leaked for a year before bursting, suddenly and cataclysmically, wiping out $6 trillion in household wealth in the process. But when it comes to the much-speculated burst of Tech Bubble 2.0, nobody’s sure what it will look like, or what kind of damage it will leave in its wake.

The latest signs of the bubble bursting could be shaky investor confidence in the private market. In the last month, conservative, high-profile investor Fidelity has written down more than a dozen of its investments in richly valued tech start-ups, Fortune reports. Dropbox and Zenefits, which recently let go of C.E.O. Parker Conrad, are among the start-ups with slashed valuations, although Fidelity evaluated Zenefits before Conrad’s departure. Other start-ups that were marked down by Fidelity include genetics start-up 23AndMe and food-kit-delivery service Blue Apron. Fidelity portfolio companies with stable valuations included, Uber, WeWork, Jessica Alba’s Honest Company, and Snapchat.

For many of Silicon Valley’s highly valued start-ups, it looks like there’s nowhere to go but down. There are now more than 150 private tech start-ups with valuations exceeding $1 billion, known as “unicorns,” with a combined valuation of more than $530 billion. Buzzier, bigger start-ups aren’t having any trouble raising funding—New York-based health-care start-up Oscar, already a unicorn, closed a Fidelity-led round of funding last week valuing it at $2.7 billion. But the start-up ecosystem is starting to contract, and this isn’t the first time in recent months that Fidelity has written down one of its own stakes in a portfolio company.

Several months ago, Fidelity whittled down the value of its stake in Snapchat by 25 percent, dropping its shares in the $16 billion messaging platform from $30.72 in June to $22.91 at the end of September.

And more broadly, there’s been a funding slow-down in Silicon Valley. Only 12 new start-ups reached unicorn status in the last three months of 2015. In the same time period, deal-making activity and the amount of venture capital being poured into Silicon Valley start-ups dropped sharply. The number of deals done in Silicon Valley decreased by 35 percent, according to analytics firm PitchBook, and V.C. funding decreased by 13 percent to $3 billion. (Nick Bilton reported on the impending collapse in an article for Vanity Fair last year.)

Mega-rounds—defined as rounds of funding of more than $100 million—have also declined. Zenefits and Jawbone, a wearables company, have laid off employees. Other start-ups, like early Lyft and Uber competitor Sidecar, have folded. Still others, like on-demand valet service Zirx, have pivoted their businesses. And last week, analytics firm CB Insights published its own Downround Tracker, a start-up death-watch list of at least 55 companies that have raised money or exited in a down round—meaning a company raised a new round of funding or sold itself for less money than it was worth to investors before.

Investors like First Round’s Fred Wilson have put pressure on heavily valued start-ups, calling for companies like Uber to go public. The public market is an equally uncomfortable place for tech companies, though: last month, there were no I.P.O.s for the first time since September 2011. And public tech companies like LinkedIn have had their valuations slashed. More sympathetic investors like Marc Andreessen downplay worries of an exploding tech bubble (at one time, Andreessen’s Twitter biography asked, “Where is the kaboom?”). But Fidelity’s start-up write-downs add fuel to the debate about whether private tech companies in Silicon Valley are overvalued. Investors are all thinking about the bubble. Now, they’re wondering how and when—not if—it bursts.