Mainak Parvat rose from the bottom of the ocean to accord rest to Hanuman. Mainak was brother in law of Shiva , was protected by lord Vayu from Vajrayudha of Indra .To repay his gratitude towards vayu he rose from ocean bed.
Mainak parvat had wings. Earlier all the mountains had wings .They out of pride ,started flying in the air and settling down at their will. This caused inconvenience to the people at large. Hence Indra cut the wings of all the mountains. But Mainak,the golden parvat ,100 yojana in width,took shelter of vayu and then consequently was saved and allowed to stay in lavan samudra.
Sagar brothers are responsible for the ocean to have ceded into the earth. thus they are called Sagar.Sagar were Suryavanshi Kings .Rama was also from the same lineage , to pay respect to the Ramdoot , Sagar ordered Mainak to rise to the occasion.
Good people always think of repaying a favour however small it may be.Mainak illustrates that. But Hanuman was not tired to take any rest. This indicates how strong Hanuman was. On a mission for the Lord, taking rest ,even when it is not warranted, is akin to wasting time and lethargy. Hanuman hence did not like to rest being a perfect servant.However to please Mainak he touched it and then flew off.
the fact that Mainak was 100yojana , 800 miles ,shows Lanka is not the srilanka we often mistake, Lanka is far away beyond malayachal ,behind trikut parvat. Srilanka is simhala dweep according to Varaha purana.