The Seventh Transept was where the Headless Monks stored the heads of those converted to their order. Most of them rotted to skulls. They were very active and ate (or at least killed) rats and intruders. The heads of richer converts, however, such as Dorium Maldovar, were stored in boxes, perfectly preserving them.
The Eleventh Doctor once went to the Seventh Transept to ask the head of Dorium Maldovar why the Silence wanted him dead. The Doctor later returned Dorium's head to the Transept. (TV: The Wedding of River Song)