The main story of the novel is the narrative of the adventures of Adam More, a British sailor shipwrecked on a homeward voyage from Tasmania. After passing through a subterranean tunnel of volcanic origin, he finds himself in a "lost world" of prehistoric animals, plants and people sustained by volcanic heat despite the long Antarctic night.
A secondary plot of four yachtsmen who find the manuscript written by Adam More and sealed in a copper cylinder forms a frame for the central narrative. They comment on More's report, and one identifies the Kosekin language as a Semitic language, possibly derived from Hebrew.
In his strange volcanic world, More also finds a well-developed human society which in the tradition of topsy-turvy worlds of folklore and satire (compare Sir Thomas More's Utopia, Erewhon by Samuel Butler, or Charlotte Perkins Gilman's Herland) has reversed the values of 19th century Western society: wealth is scorned and poverty is revered, death and darkness are preferred to life and light. Rather than accumulating wealth, the natives seek to divest themselves of it as quickly as possible. Whatever they fail to give away to wealthy people is confiscated by the government, which imposes the burden of wealth upon its unfortunate subjects at the beginning of the next year of reverse taxation as a form of punishment.

Question: How does Adam enter the 'lost world'?
Answer: Through a subterranean volcanic tunnel
Alternative answer: He passes through an underground tunnel.