Bede Historia Ecclesiastica Gentis Anglorum Book IV Chapter 19:

How Queen Æthelthryth always preserved her virginity, and her body suffered no corruption in the grave.

King Ecgfrith took to wife Æthelthryth, the daughter of Anna, king of the East Angles, of whom mention has been often made; a man of true religion, and altogether noble in mind and deed. She had before been given in marriage to another, to wit, Tondberht, ealdorman [princeps] of the South Gyrwe; but he died soon after he had married her, and she was given to the aforesaid king. —
— Though she lived with him [Ecgfrith] 12 years, yet she preserved the glory of perfect virginity, as I was informed by Bishop Wilfrid, of blessed memory, of whom I inquired, because some questioned the truth thereof; and he told me that he was an undoubted witness to her virginity, forasmuch as Ecgfrith promised to give him many lands and much money if he could persuade the queen to consent to fulfil her marriage duty, for he knew the queen loved no man more than [Wilfrid] himself. And it is not to be doubted that this might take place in our age, which true histories tell us happened sometimes in former ages, by the help of the same Lord who promises to abide with us always, even unto the end of the world. For the divine miracle whereby her flesh, being buried, could not suffer corruption, is a token that she had not been defiled by man.
She had long asked of the king that he would permit her to lay aside worldly cares, and to serve only Christ, the true King, in a monastery; and having at length with difficulty prevailed, she entered the monastery of the Abbess Æbbe, who was aunt to King Ecgfrith, at the place called the town of Coludi [Coldingham, Berwickshire], having received the veil of the religious habit from the hands of the aforesaid Bishop Wilfrid; but a year after she was herself made abbess in the region called Elge [Isle of Ely], where, having built a monastery,[*] she began, by the example of a heavenly life and by her teaching, to be the virgin mother of many virgins dedicated to God.[*] It is told of her that from the time of her entering the monastery, she would never wear any linen but only woollen garments, and would seldom wash in a hot bath, unless just before the greater festivals, such as Easter, Pentecost, and the Epiphany, and then she did it last of all, when the other handmaids of Christ who were there had been washed, served by her and her attendants. She seldom ate more than once a day, excepting on the greater festivals, or some urgent occasion. Always, except when grievous sickness prevented her, from the time of matins till day-break, she continued in the church at prayer. Some also say, that by the spirit of prophecy she not only foretold the pestilence of which she was to die, but also, in the presence of all, revealed the number of those that should be then snatched away from this world out of her monastery. She was taken to the Lord, in the midst of her flock, 7 years after she had been made abbess; and, as she had ordered, was buried among them in a wooden coffin in her turn, according to the order in which she had passed away.[*]
She was succeeded in the office of abbess by her sister Seaxburh, who had been wife to Eorcenberht, king of Kent. This abbess, when her sister had been buried sixteen years, thought fit to take up her bones, and, putting them into a new coffin, to translate them into the church. Accordingly she ordered some of the brothers to find a stone whereof to make a coffin for this purpose. They went on board ship, for the region of Elge is on every side encompassed with water and marshes, and has no large stones, and came to a small deserted city, not far from thence, which, in the language of the English, is called Grantacæstir [Cambridge], and presently, near the city walls, they found a white marble coffin, most beautifully wrought, and fitly covered with a lid of the same sort of stone.[*] Perceiving, therefore, that the Lord had prospered their journey, they returned thanks to Him and carried it to the monastery.
When the grave was opened and the body of the holy virgin and bride of Christ was brought into the light of day, it was found as free from corruption as if she had died and been buried on that very day; as the aforesaid Bishop Wilfrid, and many others that know it, testify. But the physician, Cynefrith, who was present at her death, and when she was taken up out of the grave, had more certain knowledge. He was wont to relate that in her sickness she had a very great tumour under her jaw. “And I was ordered,” said he, “to lay open that tumour to let out the noxious matter in it, which I did, and she seemed to be somewhat more easy for two days, so that many thought she might recover from her infirmity; but on the third day she was attacked by the former pains, and being soon snatched out of the world, she exchanged all pain and death for everlasting life and health. And when, so many years after, her bones were to be taken out of the grave, a pavilion being spread over it, and all the congregation, the brothers on the one side, and the sisters on the other, standing about it singing, while the abbess, with a few others, had gone within to take up and wash the bones, on a sudden we heard the abbess within cry out with a loud voice, ‘Glory be to the name of the Lord.’ Not long after they called me in, opening the door of the pavilion, and I found the body of the holy virgin taken out of the grave and laid on a bed, like one asleep; then taking off the veil from the face, they also showed me that the incision which I had made was healed up; so that, in marvellous wise, instead of the open gaping wound with which she had been buried, there then appeared only the slightest trace of a scar. Besides, all the linen clothes in which the body had been wrapped, appeared entire and as fresh as if they had been that very day put about her chaste limbs.”
It is said that when she was sore troubled with the aforesaid tumour and pain in her jaw and neck, she took great pleasure in that sort of sickness, and was wont to say, “I know of a surety that I deservedly bear the weight of my trouble on my neck, for I remember that, when I was a young maiden, I bore on it the needless weight of necklaces; and therefore I believe the Divine goodness would have me endure the pain in my neck, that so I may be absolved from the guilt of my needless levity, having now, instead of gold and pearls, a fiery red tumour rising on my neck.”  It happened also that by the touch of those same linen clothes devils were expelled from bodies possessed, and other diseases were at divers times healed; and the coffin wherein she was first buried is said to have cured some of infirmities of the eyes, who, praying with their heads resting upon that coffin, were presently relieved of the pain or dimness in their eyes. So they washed the virgin’s body, and having clothed it in new garments, brought it into the church, and laid it in the sarcophagus that had been brought, where it is held in great veneration to this day. The sarcophagus was found in a wonderful manner to fit the virgin’s body as if it had been made purposely for her, and the place for the head, which was fashioned separately, appeared exactly shaped to the measurement of her head.
Elge is in the province of the East Angles, a region of about six hundred families, of the nature of an island, encompassed, as has been said, with marshes or waters, and therefore it has its name from the great plenty of eels taken in those marshes; there the aforesaid handmaid of Christ desired to have a monastery, because, as we have before mentioned, she came, according to the flesh, of that same province of the East Angles.
Barbara Yorke, Nunneries and the Anglo-Saxon Royal Houses (2003), Chapter 1 (p.33): “We may suspect that the author of the Liber Eliensis, upon reading of Æthelthryth’s marriage to Tondberht at the beginning of the chapter of the Historia Ecclesiastica which describes the foundation of Ely, decided to make his own connection, and certainly his explanation of dower is anachronistic, and reflects late Saxon or Norman expectations of the arrangements to be made on marriage.”
Edward Miller, The Abbey and Bishopric of Ely (1951), Chapter 2 (pp.9–10): “He [the author of the Liber Eliensis] desired to establish a venerable antiquity for the privileges enjoyed by his church in his own day; and to vindicate for those privileges some sort of continuity from St Etheldreda’s time. In this respect he has been more successful than he himself may have anticipated. He has persuaded more than one historian that the medieval Isle of Ely, with its special and extensive franchise, and the modern administrative district of the Isle which is still distinguished from Cambridgeshire proper, are derived ultimately from the dower of St Etheldreda.”
The Anglo-Saxon Chronicle s.a. 673:
“St Æthelthryth began the monastery at Ely.”
The Anglo-Saxon Chronicle notes, s.a. 679, that “St Æthelthryth died”.
Cyril Hart proposes that the Isle of Ely is to be identified with another of the Tribal Hidage’s 600 hide territories: that of the West Wixna.
Ely, like Coldingham, was actually a ‘double monastery’. Double monasteries had communities of both men and women. All known examples in England were headed by an abbess.
The river Cam was formerly called the Granta. The Roman town (Bede’s ‘city’, corresponding to the cæstir element of the English name) was at, what is now called, Castle Hill (the Normans built a castle on the site) in Cambridge. The “white marble coffin” would, of course, have been a Roman sarcophagus.