Catechism of the Catholic Church §628 — excerpt
Baptism, the original and full sign of which is immersion, efficaciously signifies the descent into the tomb by the Christian who dies to sin with Christ in order to live a new life.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.