The Perilous Gard by Elizabeth Marie Pope is one of my all-time favorite GU books. I love the allusions to real folklore, and of course the passage literally under the ground. And it doesn’t hurt that the protagonist is another Kate.
SPOILERS Kate, a young woman, is banished to a faraway castle because of her sister’s stupidity. Once there, she learns the family secret: the lord’s daughter “fell” into the Holy Well and disappeared, and the lord’s younger brother was blamed for it. However, Kate slowly discovers that in fact the little girl was taken by the Fairy Folk who live beneath the well, to pay their teind to the gods. The brother, Christopher, agrees to take her place and is led underground to be kept until his death, and Kate is also imprisoned because she knows too much (and is caught by the enemy – her classic mistake). Her adversary is the Queen, the Lady in the Green, who tempts her with a drug that will erase her pain. But Kate refuses, determined to rescue Christopher before he is killed. The Queen eventually becomes almost fond of her and invites her to stay with the fairies forever, then tries to hypnotize her so she will not spoil the ritual. But Kate tricks her, finds her way through the labyrinthine tunnels, and saves the day, winning Christopher’s heart along the way.