Scientists do not yet know the cause of the recent change.

Scientists do not know exactly how they start to form.

Like me, the scientists often knew a lot before they began writing anything down.

Scientists do not know why the people left the island.

"The public came to believe that the scientists didn't know what they were doing."

The scientists still do not know exactly where the Opportunity is.

So scientists do not know how to reduce the risk.

Sleep is necessary, although scientists do not know exactly why.

It is a natural effect that scientists have known about since the 1980's.

Scientists do not know exactly how it works on the brain.