I feel sure it has not been attempted before.

As for such a swim, it's never been attempted before, the currents being what they are.

Certainly, nothing of this kind has ever been attempted before.

Many of these techniques had not even been attempted before.

But we are working in areas and making models of processes that no one has attempted before, so it's not clear what cannot be done.

But nothing like this has ever been attempted before.

Actually, I have attempted this class many times before.

Crashing is largely attempted not at the party but in the days before, by e-mail.

What we will accomplish here today has never been attempted before.

It is sure that nothing like my mission has ever been attempted before.