This piece from the first book of Preludes evokes the legend of a submerged cathedral rising from the sea. It is distinguished by its use of parallel chords and organ-like sonorities that create a sense of vast, shifting distance.
The primary challenge is maintaining a rich, bell-like tone quality and perfect control over the pedal to prevent the thick textures from blurring.
Comparable difficulty and character — good candidates for what to learn next.
La cathédrale engloutie is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of La cathédrale engloutie lasts about 6 minutes.
La cathédrale engloutie is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Canope by Claude Debussy, Berceuse héroïque, CD 140 by Claude Debussy, Clair de lune by Claude Debussy — see the full similar-pieces list on this page.