Some people know (but mostly – not), what happens underground or in the depths of a mountain. For instance, underneath a lush chunk of forest there might flow a subterranean river as a underwater channel between two lakes. Fish can use it to travel to some secluded nooks. Dive deep in their home lake surrounded by meadows with mountain goats and swim through a dark little undulating tunnel which ends in a forest lake or even in some sort of secretly hidden oasis between ridges looking like a plain mountain from the outside, but tucking away a meadow inside it. Like a tooth that seems externally healthy but hides nothing but decay within.
Most likely, these kind of things never existed, but I would love for at least this one instance to be real: a creek flowing down the mountain plunges deep into its depths and travels to the lake in small streams. This lake has an underground river leading to a smaller lake lying below and runs further to a second, very deep one.
70 x 89 cm