St. Thomas Aquinas in Ep. ad Gal. cap VI.
About the Crux Angelica:
One night in June, 1228, a lightning stroke smote the tower in which the child of grace [St. Thomas Aquinas] lay sleeping beside his nurse: in agony of mind the alarmed mother ran to the spot, to find him unharmed, while her little daughter lay dead and charred, and the horses in the stables beneath were killed. This occurrence left in him a life-long nervousness and dread of storms, which he could never allay. In consequence of this, in later years in a subterranean cave at Anagni, he traced upon the walls in capital letters this distich in fashion of a cross.
From a life of St. Thomas by Fr. Placid Conway, OP
To read the Crux Angelica start from the center. The word CRUX goes up, down and left and right and the text continues all the way in each direction and then across the top, bottom, and sides. Then the prayer begins again from the middle at the next line. At the end of each bar of the cross some words share letters of the previous or next line. At first glance this is very confusing but once you get the plan it is indeed an amazing work of faith.
CRUX MIHI CERTA SALUS
CRUX EST QUAM SEMPRER ADORO
CRUX DOMINUM MECUM
CRUX MIHI REFUGIUM