When Hanuman was searching for Sita, and was flying over the sea. That time sea asked Mainaka (मैनाक) to rise up out of the water and give Hanuman a place to rest a while.
Hanuman’s father Vayu Devata once saved Mainaka from the anger of Lord Indra and made him hide in the sea.
This episode is mentioned in Valmiki Ramayana Sundara Kanda First Sarga verse 122-126
पूर्वं कृतयुगे तात पर्वताः पक्षिणोऽभवन्।
ते हि जग्मुर्दिशः सर्वा गरुडानिलवेगिनः।
“In Kritayuga, mountains were having wings. They used to move speedily in all directions like Garuda and the Windgod.
ततस्तेषु प्रयातेषु देवसङ्घा: महर्षिभिः।
भूतानि च भयं जग्मुस्तेषां पतनशङ्कया।
“When they moved around it created the fear and panic in hearts of sages, gods and creatures
ततः क्रुद्धः सहस्राक्षः पर्वतानां शतक्रतुः।
पक्षान् चिच्छेद वज्रेण तत्र तत्र सहस्रशः
“Provoked at this, Indra cut off the wings of mountains into a thousands pieces with his Vajra.
अस्मिन्लवणतोये च प्रक्षिप्तः प्लवगोत्तम।
गुप्तपक्षसमग्रश्च तव पित्राभिरक्षितः।
“O distinguished Vanara ! your father, saved my wings by throwing me down into this sea. Thus my wings were protected.
Surasa (सुरसा) the mother of the nagas (serpents). She was asked by devata to test the Lord Hanuman on his way to Lanka. She adore a form of rakshasi and asked Lord hanuman to enter her mouth.
She mentioned that she has a boon that no one can cross her region.
एवमुक्ता हनुमता सुरसा कामरूपिणी।
अब्रवीन्नातिवर्तेन्मां कश्चिदेष वरो मम।
Having heard Hanuman, Surasa capable of taking any form told Hanuman, that she has a boon that no one can cross her.
She tells him that he can pass only through her mouth, as per a boon given to her. Hanuman counters her by challenging to open her mouth wide enough to eat him . Hanuman starts expanding his form and become larger and larger, Surasa also expanded her jaws so she can hold Hanuman.
Finally when Surasa’s mouth was expanded to 100 yojanas, Hanuman suddenly assumes a tiny form (size of a thumb) and enters her mouth and leaves it, before she can close it.
Simhika (सिंहिका) is a rakshasi in Hinduism.
After meeting Mainaka and Surasa, when Hanuman was crossing the ocean to Lanka.
Simhika who was hiding in the ocean. She captured the Hanuman’s shadow with her power and increased her mouth to eat him.Hanuman turned himself small and allowed her to swallow him, ripping her apart from within, killing her before resuming his journey.
तद्धृष्ट्वा चिन्तयामास मारुतिर्विकृताननम्।
कपिराजेन कथितं सत्त्वमद्भुतदर्शनम्।
छायाग्राहि महावीर्यं तदिदं नात्र संशयः।
Seeing the form of rakshasi, Hanuman thought that “King Sugriva has told him about a creature in the sea who can capture Shadows. it seems the same to me”