SB 4.25.2

rudra-gītaṁ bhagavataḥ
 stotraṁ sarve pracetasaḥ
japantas te tapas tepur
 varṣāṇām ayutaṁ jale
Word for word: 
rudra-gītam — the song sung by Lord Śiva; bhagavataḥ — of the Lord; stotram — prayer; sarve — all; pracetasaḥ — the princes known as the Pracetās; japantaḥ — reciting; te — all of them; tapaḥ — austerity; tepuḥ — executed; varṣāṇām — of years; ayutam — ten thousand; jale — within the water.
Translation: 
All the Pracetā princes simply stood in the water for ten thousand years and recited the prayers given to them by Lord Śiva.
Purport: 

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.