A group of demons around Vritra who hid in the ocean.
Sage Agastya drank the ocean empty so that they could be destroyed. Then he filled the hole with his urine.
This became a problem in the Ramayana when it was necessary to get to Lanka to free Sita from the hands of Ravana. Nobody wanted to swim in Agastya’s urine, nor did they want to ask him again to empty the ocean and drink his own urine. So the only option was to build a bridge with great effort.