In one version of Ramayana, Lord Hanuman visited Patala loka to save Lord Rama. But how did he find way to Patal loka?
The story is told in the Krittivasi Ramayana, a Bengali version of the epic. To protect Rama and Lakshmana from Ravana's son Mahiravana, Hanuman makes a fortress out of his tail and puts them inside, with the top of the fortress being guarded by Vishnu's Sudarshana Chakra. Vibhishana tells Hanuman not to let anyone in, not even his own father, because Mahiravana can disguise himself as anyone. But then Mahiravana disguises himself as Vibhishana, and Hanuman lets him in. After some time Hanuman realizes that wasn't Vibhishana, and so Hanuman and the real Vibhishana go into the fortress
"I am filled with dread. Has Mahiravana with his magic gone within the fortress?" thought Hanuman as he bowed his head in shame.... "Listen Vibhishana," Hanuman said, "let us go within to see Rama and Lakshmana." "Let us go," said Vibhishana. Both of them rushed in and to their surprise they saw a tunnel which was not there before.
Then Hanuman goes into the tunnel:
Then he wept aloud "Alas! When shall I see Rama and Lakshmana? Remain together all of you in one place till I return." Bowing to Sugriva he took his leave and entering the tunnel, was lost to sight. Inside the tunnel he went on and on, till he came to the Under Earth World. Here he found a sunny land, with flowing rivers and beautifully designed palaces and most lovely women. He saw holy men in their forest retreats busy with their sacred books and prayers. He went further and met many kinds of people, some had four hands, and there was no sickness nor sorrow, old age nor death. Many amazing countries he passed through but could not find any trace of Rama and Lakshmana. One by one, he searched every house until he came to the palace of Mahiravana.
What happens after that is well-known. Mahiravana wants to sacrifice Rama and Lakshmana to the goddess Kali, and tells them to bow to the Kali statue. They say they're not accustomed to bowing, so Mahiravana shows them how to do it and Hanuman takes the opportunity to cut off Mahiravana's heads. He then fights off Mahiravana's wife and son Ahiravana, and then Rama, Lakshmana, and Hanuman leave Patalaloka.
Note that this story is not mentioned at all in the Valmiki Ramayana, only in much later versions.