It's a park! It's a lake! It's... both?
Just below Austria's Hochschwab mountains near the town of Tragoess sits Grüner See ("Green Lake" in English).
Why do I care, you say? Well, you care because it literally changes with the seasons -- and we're not just talking about leaf colors. During winter, the lake is only three to six feet deep and is surrounded by parkland that is perfect for hiking and exploring.
Winter, for example: