A new wave of Twitter employees resigned Thursday after Elon Musk issued an ultimatum telling them they would need to be willing to commit to a “hardcore” work environment.

Internal Slack messages shared with CNBC showed engineers and other employees posting goodbye messages to a “watercooler” chat group in the run-up to the 5 p.m. ET Thursday deadline Musk set Wednesday. 

Hundreds of salute emojis (which convey the message “thank you for your service”) streamed by, along with dozens of goodbye messages.

Three Twitter employees who spoke with CNBC asked to remain anonymous, citing fear of professional retaliation. All three were planning to resign Thursday. It was not clear exactly how many Twitter employees resigned. 

“The train has started in #social-watercooler” one of the employees said, referring to a Slack room Twitter employees have used in recent weeks to notify others that they are leaving.

Musk sent a companywide email Wednesday telling employees to expect “long hours at high intensity” if they wanted to stay. He said they had until 5 p.m. ET Thursday to decide. 

Musk followed that up Thursday with a pair of emails that said managers must meet with employees in person once a week or at least monthly and that managers could be fired for allowing employees to work remotely if those employees do not prove, in his view, to be “excellent” or “exceptional.”

Nov. 14, 202200:37

Musk has asked some top engineers who opted to resign to consider staying on, said a Twitter engineer familiar with the situation.

The recent wave of resignations adds to what is now a combined mass layoff and voluntary exodus from Twitter, leaving the company significantly smaller than when Musk took it over late last month. 

An engineer said resignations had hit important parts of the company’s engineering operation.

“Entire teams representing critical infrastructure are voluntarily departing the company, leaving the company at serious risk of being able to recover,” the engineer, who said they were handing in their resignation Thursday, wrote to CNBC.

The engineer added that many leaving Twitter did not feel the need to stay and that they knew of only two people staying, one because the company sponsored their U.S. visa.

“We are skilled professionals with lots of options, so Elon has given us no reasons to stay and many to leave,” they wrote.

Esther Crawford, who works on early-stage products at Twitter, sent a farewell message to those leaving the company.

“To all the Tweeps who decided to make today your last day: thanks for being incredible teammates through the ups and downs,” she tweeted. “I can’t wait to see what you do next.”

Later Thursday, the company said it would be closing all offices until Monday, according to an email a departing employee shared with CNBC.

“Please continue to comply with company policy by refraining from discussing confidential company information on social media, with the press or elsewhere,” the email added.