Lake Hylia

Lake Hylia is the name of a fictional area in the Zelda series. In most games, the lake is fed by a river which flows from a waterfall which flows from a mountain (usually Death Mountain).

The Legend of Zelda
Though it is never officially named, many fans of the series theorize that the lake in central Hyrule on which Level 1 and Level 4 are located is Lake Hylia. This is due in part to its resemblance to Lake Hylia in other games. Though the area around the lake is inhabited by various creatures, the lake itself seems to be inhabited solely by Zoras.

A Link to the Past
Lake Hylia is in southeast Hyrule and has small caves coming off it. In the center of the lake is an island featuring a fairy pond that is inhabited by Venus, Queen of the Fairies, who will upgrade your ability to carry bombs or arrows if you give her enough money. Outside the pond is a warp tile that grants access to the Ice Palace in the Dark World.

The lake itself is fed from Zora's River, which in turn flows from Death Mountain.

Ocarina of Time
Lake Hylia is a lake in the south of Hyrule. It is closed off by a gate but can be entered on Epona or by scaling a ladder hidden behind one gate pillar. There is also portal from Zora's Domain and it is possible to float down river from Gerudo Valley. Lake Hylia is fed by water from Zora's River. There are numerous small islands in the lake linked by bridges. The largest island is the warp point for the Serenade of Water and below it lies in the Water Temple. Also in Lake Hylia, there is a laboratory and a fishing house.

Geology
Though Lake Hylia in A Link to the Past is a fictional lake, aspects about it suggest it is a maar lake, a kind of volcanic cone. The evident details include: It is not known whether Lake Hylia is or was ever intended to be a maar or not.
 * Hyrule has other volcanic activity nearby, such as Death Mountain, establishing volcanism in the area.
 * The lake is small, round and very deep, typical of a bowl-shaped maar.
 * Some of the lake edges are surrounded with steep walls, typical of maar formations, formed by a maar's initial bowl-shaping eruption.
 * The lake's faerie spring island could conceivably be a man-made (or faerie-made) degassing pump, to prevent carbon dioxide from oversaturating the lake water. Such pumps work to fountain water from the bottom of the lake to the top, expelling excess gas before it can build up at the bottom.  Though real degassing pumps (such as at Lake Nyos) are fountains that spray water from the lake bottom up into the air, the noted absence of a fountain at Lake Hylia could mean either that:
 * The maar is inactive. Considering Ocarina of Time and the presence of fish in Lake Hylia, this is likely.
 * The faerie spring decarbonates the water and releases the gas into the air. This is less likely, as carbon dioxide sinks in atmosphere, and would blanket the surrounding ground and lake surface with a thin layer of unbreathable air.  Swimming would be impossible.  This assumes that the faerie spring doesn't teleport the gas to a separate location.