The ship is under weigh.

Under no circumstances should you give out your personal information to strangers.

There is nothing new under the sun.

The boat got caught in an under tow and was pulled out to sea.

The document was filed under seal.

He goes under the bridge.

She is going under the water to search for the lost treasure.

She has gone under the water to search for her lost necklace.

She went under the water to retrieve the lost ring.

I hate pop-unders, they are so annoying.