Elon Musk's Twitter tenure faced a new level of crisis Thursday evening as a large number of employees apparently chose to quit rather than accept the new owner's ultimatum to sign up for "extremely hardcore" conditions with "long hours at high intensity."

By the numbers: There's no way to know for sure how many employees remain at Twitter, but Musk had already laid off roughly half the social media company's 7,500 workers.

• Multiple reports suggest the additional Thursday resignations number at a minimum in the hundreds, and include a high proportion of the engineers responsible for maintaining the service.

Why it matters: The further reduction in staffing has raised fears about the service's short-term reliability — particularly since it comes on the eve of the World Cup soccer tournament, during which heavy usage typically stress-tests Twitter's systems.

What's happening: Musk met Thursday afternoon with a group of workers whose roles were considered critical in an effort to persuade them to stay, the New York Times reports.

• As the day advanced, employees who'd opted to leave signed off on the company's Slack message board using salute emojis.
• Many Twitter users also began saying goodbye to one another — even though, for the moment, the service appears to be mostly functioning normally.

Between the lines: Many current and former employees predicted to multiple news outlets that Twitter's brain drain would inevitably lead to system failure.

• The Washington Post reported that the latest resignations had left key teams directly tied to the service's stability with no remaining members.
• Twitter employees believe it is just a matter of time before the service encounters major technical problems, per The Verge.

The intrigue: Twitter sent an email late Thursday to remaining employees telling them that the company's offices would be closed and badge access restricted through Nov. 21, Bloomberg reported.

• The move echoed a similar shutdown during the initial round of layoffs earlier this month.

Of note: Musk sent a confusing series of communications to Twitter staff on Thursday about his mandate to end most remote work at the company, per the Times.

• On the one hand, Musk told employees that all they had to do to continue working remotely was for their managers to vouch for the excellence of their work.
• But then, in a followup email, he said he'd fire any manager who "falsely" claimed a worker's performance was excellent, according to the Times.