Oceans worth of Tears
You get used to the rain after... two... twenty years. Lets go with twenty.
A large lake sits amongst the caverns of the Subterrane. With thundering rainfall, it is the largest body of water in the depths, known as the Crying Sea.
People make both their homes near its shores and trade amongst one another. Although there are also legends buried beneath its rippling waters that only a few natives know about.
A Rainy Sea
Because of its position beneath the Crescent Sea, water seeps through the cave’s roof. Over millennia, this carved the cavern this body of water now resides in. And due to the sheer quantity, results in a damp lake with constant downpour from its stalactite roof.
From it comes an underground sea where rain falls down upon anything sailing upon it. Any overabundant water flows down into the lower caverns of the Subterrane.
Life beneath the ripples
Few creatures live near the surface of the water, being startled by the constant downpour. Instead, they live much deeper, amongst dark depths where little light is present.
Amongst them are a lot of cavefish and other forms of aquatic troglobites. The number of species go somewhere around the hundreds, though people assume there to be much more than they know. From small bottom feeders to fish the size of boats.
Civilisation on Pillars
Many people settle near its shores, though some also go further in. Through untold, large pillars formed across the sea, providing some semblance of land. To shorten the journey from one side to the other, people began setting up ports around them.
This led to the Pillar Folk, who build towns on stilts and maximise what little space they have. Their people trade amongst one another, surviving on what merchants offer and what the sea provides. All forming a culture under the rainy sky of the caverns they live in.