The Endless Well (5e)

From the introduction: "The Endless Well" is a short D&D adventure for four to five 5th-level player characters (PCs). The scenario takes place in the World of Farland, specifically in the "Endless Well," a series of caverns linked through a portal to water energy in the Maelstrom (or the elemental plain of water in other D&D worlds). It is the source of a river that never dries, even in times of drought.