The Underground Caverns are accessible from the lower-left section of the Marble Gallery or by falling through a specific floor in the Castle Entrance. This area is characterized by its water-filled passages and aquatic enemies like Fishmen and Frogmen. The Holy Symbol is essential here to avoid taking damage from spiked water pits. Use your Bat form to navigate large open areas and reach high ledges.
Your primary objective is to find the Merman Statue relic. This item allows Alucard to summon a Merman familiar, which can attack enemies and retrieve items from water. It's located in a hidden room, often requiring the Form of Mist to pass through a grate. There are multiple Save Rooms and a Teleporter in this extensive area. explore thoroughly, as there are many hidden items and passages.