Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 2


रुद्रगीतं भगवत: स्तोत्रं सर्वे प्रचेतस: ।जपन्तस्ते तपस्तेपुर्वर्षाणामयुतं जले ॥ २ ॥


rudra-gītaṁ bhagavataḥstotraṁ sarve pracetasaḥjapantas te tapas tepurvarṣāṇām ayutaṁ jale


All the Pracetā princes simply stood in the water for ten thousand years and recited the prayers given to them by Lord Śiva.


Of course in the modern age one may be amazed how the princes could stand in the water for ten thousand years. However, living within air or living within water is the same process; one simply has to learn how to do it. The aquatics live within water for their whole life span. Certain favorable conditions are created to enable them to live within water. In those days, however, people used to live for one hundred thousand years. Out of so many years, if one could spare ten thousand years for the sake of austerity, he would be assured of success in his future life. This was not very astonishing. Although such a feat is impossible in this age, it was quite possible in Satya-yuga.