The Romans Doxology: "Immeasurable Depth"

The Apostle Paul has spent the first 11 chapters of his great Epistle to the Romans scaling the mountain of the great theological realities of God's way of making sinners righteous. When he reaches the peak in Romans 11:33, he looks back down from where he has just ascended and can only exclaim, "O the depth of the riches and the wisdom and the knowledge of God!"

What has prompted this ascription of praise to our sovereign God?