"Across the Sea" is the 15th episode of the sixth season of the serial drama television series Lost and 118th episode overall. The episode aired on May 11, 2010, on ABC in the United States. The episode was written by Carlton Cuse & Damon Lindelof and directed by Tucker Gates. The episode is centered on Jacob and The Man in Black. This episode is the second in the season that does not feature a "flash-sideways".

The episode, unlike most Lost episodes, does not divide its time between two different time settings, but takes place entirely in the past, except for a brief scene at the end from the season one episode "House of the Rising Sun". It depicts the origins of the characters of Jacob (Mark Pellegrino) and his brother, The Man in Black (Titus Welliver), and how they came to be on the island. It also reveals the identities of the corpses discovered in the cave in "House of the Rising Sun". It is the only episode of the series in which none of the series regulars appears, except in the flashback sequence.

This is the only episode where the Man in Black appears in his original, human form. In all of his other appearances, he is either in his smoke form or in the form of a dead person. Although he appears in his own form in "The Incident" and "Ab Aeterno", this was an imitation of his original form which by that point was dead.

This episode is notable for receiving a widely mixed opinion, with many commentators alternately considering it one of the best or worst episodes of the series.

Washed ashore on the island, a pregnant woman named Claudia (Lela Loren) encounters an unnamed woman (Allison Janney). The woman helps Claudia give birth to her son, who Claudia names Jacob. Another boy is unexpectedly born. The woman kills Claudia and raises the twins as her own, keeping them ignorant of the outside world. She teaches them to be distrustful of humanity, which she regards as corrupt and dangerous.

At age 13, Jacob's brother (the future Man in Black) finds a Senet game. Jacob (Kenton Duty) asks about the box and his brother (Ryan Bradford) asks him to keep it secret, but the woman (Mother) sees through Jacob. She confronts Jacob's brother and claims she left the box for him, telling the young boy that he is "special". When the boy persists in asking about their origins, Mother explains that she came from her own mother, who is now dead, adding that the boys will never need to worry about death.

After following a boar, Jacob and his brother discover other people on the island. Mother warns the brothers to avoid the other people; she reluctantly decides to show the brothers a tunnel filled with light and an underground stream. She claims that one of them will eventually be the protector, and that she has made them incapable of harming each other. Mother explains that the light is highly coveted and warns that "if the light goes out here, it goes out everywhere."

A vision of Claudia appears to the young Man in Black. She reveals herself as their birth mother, and claims the other people came to the island with her. The young Man in Black confronts Mother about Claudia's claims, and attempts to convince Jacob to join the other people with him. However, Jacob refuses and stays with Mother.