Underland appears mainly in The Silver Chair, where Eustace Scrubb, Jill Pole, and Puddleglum travel under the ground to reach it in their search for Prince Rilian.
From the high vantage of the Giants' house, they see that the trenches are in fact letters, and recognize the phrase "Under Me" as one of the signs given by Aslan to guide their quest for Rilian.
[5] The last stage of their journey takes them on board a ship, which is rowed across the vast underground Sunless Sea to the city where the Dark Castle lies.
As Golg, their Earthman guide, leads them past the opening to Bism, the travelers look down into it and make out a river of fire where Salamanders swim, alongside groves and fields of many colors, not unlike a stained glass window.
[6] After Caspian X's death, Rilian has been crowned the new king of Narnia, and the Narnians use their magic to lower the water to its original level, which becomes a tourist attraction.