unpierceable

It was a very trying experience, as the river was absolutely unknown to me; the darkness was "unpierceable by power of any star," and the river was treacherous in itself for small boats.

Thus, in the traditions of every ancient nation, there is a vast and misty tract of time, expressed, if at all, in figures of appalling magnitude—hundreds of thousands, nay, millions of years—between the unpierceable gloom of an eternal past and the broad daylight of remembered, recorded history.

Its edge did not, like that of their own weapons, blunt with usage, and they could well understand that, if armor could be formed of it, it would be altogether unpierceable.

The iron line of the British was unpierceable!

"Estelle is a courageous woman and discreet with the unpierceable reticence of spontaneity."