Guruvayur became divine also on account of the "tapa" performed by Lord Siva and later by the Pracetas (the ten sons of Pracinabarhi). This was done inRudratirtha – the sacred tank on the northern side of the present temple.
Pracetas came to this place to do penanceto Lord Vishnu. Sensing their desires, Lord Siva emerged out of the Rudratirtha and spoke to them the "Rudragita", a hymn in praise of Lord Vishnu. Siva suggested them to chant it with all their heart to get their wishes fulfilled. The Pracetas won the favourof Lord Vishnu after rigorous tapas for 10,000 years under the waters of Rudratirtha chanting Rudragita. Later they married and begot as their son Prajapati Daksha.
More details are given inSrimadBhagavatam canto 4 chapter 30.
It is said that in the ancient days, the pond extended up to Mammiyur and Thamarayur (about 3 km away from the Temple).