The following historical passage was written by Fr. Seraphim Thipolos in 1548 (7056 since the foundation of the world), who was Protos on Mount Athos, and this story was handed out to us by St. Nikodimos the Hagiorite [of Mt Athos].

Nearby the Skete of Protaton, which is located in Karyes of Mt. Athos- at the left bank of the river Livadogene, below the Russian Skete of St. Andrew – near the area of the Holy Monastery of Pantokratoros, there is a big pit where various cells are also founded there. In one of those cells which was dedicated to the Dormition of the Mother of God, a pious old hieromonk used to live with his obedient disciple. As it was a custom to have a vigil each Sunday at the Skete of Protaton, during a Saturday evening the Elder wishing to go to the vigil said to his younger monk:

The appearance of Archangel.

-I, my child, will go to attend the vigil as usually. You may stay at the cell and read the service.

During the night, the young monk heard a knock on the door. He opened up and saw a stranger, an unknown monk, who asked him to come in. The stranger spent that night at that cell.
Around dawn, they woke up and chanted the hymns. When they were about to chant the Magnificat, the young monk said the well known hymn which was composed by St. Cosmas the Poet, while the unknown monk stood in front of the icon of the Theotokos. He, then, with much piety and fear started chanting in a sweet tone, beginning though, differently the chant:

“It is truly meet to bless thee, O Theotokos,thou the ever blessed, and most pure, and the Mother of our God”.

After that he chanted the well-known Magnificat till its end. When the young brother heard that, he was fascinated because it was something new to him and also because it sounded so angelical and as a celestial melody. He then turned to the unknown monk and said:

-“We chant plainly the Magnificat. This “Axion Estin” has never been heard to us or to those who lived before. Please, do me a favour and write down this hymn for me, so I may chant it to the Theotokos”.

– “Fetch me ink and paper and I will write this hymn,” he replied.
– “I have no ink nor paper,” said the young monk.
Then the mysterious guest said to him:
– “Fetch me a stone slab.”

He went and brought him a stone slab which was probably part of the floor of the church. It was then taken by the stranger and he wrote the hymn of “Axion Estin” on it with his finger. Miraculously, the letters were so deeply engraved on the hard slab as if they were written on a soft mud. Who could describe the surprise of the dumbfounded monk staring at this marvel while receiving the stone?

Then the stranger said to him:

-“From henceforth you too should chant this hymn to our Lady the Theotokos likewise, as well as all the orthodox.”
He then vanished . It was Archangel Gabriel sent by God to reveal this angelic hymn to mankind. The monk, who was extremely astounded and joyful, fell down on the place where the Angel stood and exclaimed: “Now I know truly that the Lord sent to me His Angel.” Gazing at the icon he added: “Glorious things were said concerning you, our Lady Mary, O city of God!” (Ps.86(87):3)

At the Protaton.

When the Elder returned from the vigil back to the cell, the monk began to tell everything that had happened and chanted the “Axion Estin”, as the Angel had instructed him. Moreover, he showed him the tablet with the Angel’s letters. The Elder, who heard and saw all these, was deeply surprised with the miracle. Both of them took that stone and went to the Protaton. It was shown to the Protos and to the Elders and they told them everything. They glorified God and thanked Lady the Theotokos for this special happening. They immediately sent the stone slab to the Patriarch of Constantinople and to the Emperor with letters which described the whole event.

Pit of “Adein”.

Since then, this angelic hymn has been spread all over the world and  it is chanted to the Mother of God by all Orthodox. Her icon which was at the cell where the miracle took place was taken to the Sanctuary of the Protaton, where it stands till nowadays as a royal throne following a unanimous decision of the fathers.
And that cell was renamed “Axion Estin”, while the whole area around there is called “Adein”, because at that place this angelic hymn was firstly sung.

The liturgical book of Menaion (11th of June) witnesses the fact that the angel who had appeared as an unknown monk was the Archangel Gabriel. It says: “This day, we commemorate the feast of Archangel Gabriel at Adein”. Since that miracle happened on the 11th of June – late of the 10th c.- the fathers began to gather annually and have the D. Liturgy at that pit which was renamed as “Adein”, in remembrance of the miracle, honoring Archangel Gabriel. The Archangel had been a divine chanter of the Mother of God , Her nourisher, servant, and joyful messenger of glad tidings to Her.

(www.keliaxionestin.com)