Lord Rama said: “Sugriva we have arrived at the southern shore of this great sea. I am again tormented with the diffidence which visited me before. This lord of the rivers has his other shore too far away from here. It is not possible to cross the sea unless we hit upon a method soon. Let the army call a halt here now. Let us sit and think up methods of crossing this wide ocean.”
The next reference of building the bridge across the sea occurs in the sixth chapter of the Yuddha Kanda. Now, Vibhishana, the brother of Ravana, has jumped sides and is now helping Lord Ram.
Vibhishana suggests that Lord Ram should ask the favor of the lord of the seas. Lord Ram agreed to the suggestion and asked the opinion of Lakshmana and Sugriva.
They said: “We feel too, along with you, that the advice of Vibhishena is sound. The sea can never be crossed unless there is a bridge to span it…. Pray to the lord of the oceans and he is sure to let us cross the sea.”
Lord Ram spread the darbha grass on the shores of the ocean and he meditated upon the Lord of the Seas. But the Lord of the Seas did not notice Lord Ram and his Prayopavesha. Lord Ram lost his patience and decides to drain the ocean with the dreaded Brahmastra (arrow).
Terrified, the Lord of the Seas appears before Lord Ram.
He said: “….The earth, the winds, the sky, water and fire are all ruled by the laws of nature and they should not be overstepped. I am by nature deep and wide and it is not possible to cross me. That is my nature and it cannot be altered. If the bottom is exposed then it will be against all the laws of creation. … That is the reason why I was silent when you asked me to let you cross the sea, pass over the waves which are part of me.”
Lord Ram was pacified and on the request of the Lord of the Seas, he dispatched the Brahmastra towards Dramakulya to kill the demons polluting the sea waters in that region.
Lord of the Seas then gave an idea to cross the sea.
Samudra spoke to Ram: “I will tell you how you can cross me with your army. This Vanara (monkey) with you, Nala by name, is the son of Vishvakarma, the architect of the devas. He has inherited his father’s craftsmanship and he is very clever in the art of building. Let him build a bridge across me and I assure you, I will take good care of it. It will not sink now will it come apart when this huge army marches on it.”
Thus saying Samudra vanished.
Then Nala said to Ram: “I will build a bridge across the sea. I have the skill of my father in me and I assure you, I will make the bridge wide and strong. It is true what Samudra said. I can do it.”
According to the instructions from Nala the monkeys in the army hurried to collect the materials needed for the bridge. All the trees around were uprooted and piled on the shores of the sea. The mountains were denuded of their peaks and their trees. The rocks which were like elephants were carried without any effort by the many Vanaras….
Hanuman joined the rest of them and he wanted to serve Ram by assisting in the work done by the monkeys. He would fling a large boulder and Nala would catch it with his left hand so the work proceeded with great speed.
Fourteen yojanas was built on the first day, twenty on the next. (Yojana is a vedic measurement and is between four and nine miles.) The bridge was completed in five days and the mountain by name Suvela formed the other end of the bridge.
The bridge built by Nala was beautiful and it was poised in the middle of the sea looking like the Milky Way which is woven across the sky. To the celestials, looking from the sky, the bridge seemed like the parting in the dark hair of a woman.
Lord Ram was the first to set foot on the bridge and he was followed by Sugriva and Lakshman. And the army followed.
Source: Ramayana by Kamala Subramaniam (Bharatiya Vidya Bhavan)