Right now, anyone who joined Facebook in the last 5 years is underwater on their options.
If you factor in taxes, you could end up owing more in taxes than the shares are worth (ex: vest price is $10/ea, taxes are $3/ea, current price you can sell at is less than $3 == underwater).
I think Aurora employees were locked out for 180 days, but force to exercise options at IPO price.
[0] - https://www.cnbc.com/2020/08/28/nearly-200-uber-employees-su...
Never mind the convoluted tax implications of exercising options that are nominally not underwater, but whose stock has a lower value that at the time of granting.