The oldest portions of the crypt, and therefore of the Cathedral, that remain to us are to be found in the Martyrium or Chapel of S. Lubin. The crypt, it must be understood, was not in origin a crypt or a martyrium or a meeting-house of prayer dug beneath the level of the soil, but a tiny church set on the crest of the hill, and raised above the surface of the earth. It only became a crypt properly so-called when it had been covered up and the surrounding soil raised by the _débris_ and deposits of succeeding years; so that when the new church was built it was erected naturally upon the top of the old. The spade of the archæologist has proved that the soil of a mediæval European town was raised by the accumulation of dust and rubble as much as one or two feet per century. And at Chartres excavations have revealed Gallo-Roman sub-structures at a depth of some eighteen feet. When we reach the Martyrium or Chapel of S. Lubin we shall find there a piece of the Gallo-Roman walls of the fourth century. This probably is part of the apsidal wall of the church of that date. As the church, with the vicissitudes of the town which we have yet to recount, was alternately built and burnt, destroyed and rebuilt, the crypt was developed until it is now the largest in France, and next to that of S. Peter’s at Rome, and that of Canterbury, the largest in the world. It consists of two lateral galleries which run from the western towers under the aisles of the upper church, and form a horse-shoe curve beneath the choir and sanctuary 366 feet long, and seventeen to eighteen feet broad; of two transepts, seven apsidal chapels, and the _martyrium_ which is under the choir of the upper church.
The successive developments of the crypt may be summarised as follows:--
Against the fourth century apsidal wall, of which traces are found in the martyrium, were built in 858 two large columns to support the new choir above. At the same time the circular wall of the apse was pierced with windows. In order to support the apse of the upper church other two large isolated piers were built in 962, whilst the windows in the circular apse were blocked by a second strengthening wall. The same year saw the addition of a double transept at the commencement of the apse. Fulbert, in 1020, developed these transepts by carrying them out westwards almost to their present extent, and by so doing he left the altar dedicated to the Virgin, though unmoved, no longer before a thick wall, but stranded as it were in a corridor--a situation which aided, and still aids, the progress of the pilgrims past the shrine. The great bishop also extended the crypt by piercing the wall which closed the transepts towards the east, and making the ambulatory out of which opened the three large chapels with deep, round-headed windows (S. Joseph, S. John the Baptist, and S. Anne). To support the vaulting of this ambulatory a new wall was built round the martyrium.