User:Dermotmacflann/Theories
My theories.
Lake Hylia as a volcano
Lake Hylia may be a volcano — not necessarily an active volcano, but a volcano nonetheless. But unlike the complex volcanism of Death Mountain, Lake Hylia is most likely a maar. Evidence for this includes its maar-like punchbowl shape — vaguely circular shoreline, steep lake walls, and great depth, and the fact that its greatest depth at the center goes down even deeper below the lakebed into the Water Temple, a cross-section of which would probably resemble the remnants of a volcanic pipe.
It is possible that Lake Hylia is still active, but that its largely limited to limnic eruptions. In a limnic eruption, the deepest waters become increasingly saturated with carbon dioxide gas from its volcanic source. When the deep waters become too saturated, the lake overturns and releases all the gas at once. In the real world, such an eruption at Cameroon's Lake Nyos in 1986 destroyed surrounding villages, killing 1700 people and 3500 livestock. After that disaster, volcanologists started installing deep water pipes to continually fountain the gas so it doesn't have a chance to oversaturate the water.
It may be that Lake Hylia's Water Temple and/or Lanayru Spring already serve this function, as the lake's water is already constantly filtered to keep it pure. It remains pure enough to supply Castle Town with drinking water. If Lake Hylia were active and limited to limnic eruptions, this would effectively render the volcano inactive as it would be unable to erupt either way.
Hyrule as a rift valley basin
If indeed both Death Mountain and Lake Hylia are volcanoes, it may be safe to guess that much of Hyrule's geology is of volcanic origin. Additionally, its appearance and layout in Twilight Princess gives the impression that it is made up of horst and graben. A graben is an area of land subsidence caused when the crust spreads and becomes thinner. In places like the Great Rift Valley and in Iceland, this creates subsiding volcanic lowlands nestled against swollen volcanic highlands.
There is evidence in Twilight Princess that such a process could have progressed somewhat since Ocarina of Time. Indeed, Hyrule's landscape is shorn by gaping chasms, and the floor of both Zora's River and Lake Hylia appear to be sinking to lower elevations relative to the surrounding countryside. This could have influenced the change in course of Zora's River since Ocarina of Time; rather than the river changing course and rapidly cutting a deep gorge, the gorge may have opened first and the river started flowing into it. Lake Hylia may have genuinely formed as a volcano, but is also located in a sinking graben.
If Hyrule is a rift valley basin, it is also possible that most of its land is significantly below sea level, as seen in other rift basins located inland from the sea. When the Great Flood occurs in the Adult Timeline, it may not have been rain alone that inundated the kingdom, but the break of a natural dam that allowed the ocean's waters to fill the basin, leaving only its highest points as islands in the Great Sea. Magic alone kept Hyrule below dry and frozen in time. Magic aside, this is not without precedent, as the entire Mediterranean Sea has dried up and refilled multiple times in ancient prehistory in a similar manner. However, during these dry periods (part of what is called the Messinian salinity crisis), the Mediterranean Basin was mostly a salty desert, not unlike Death Valley (or perhaps the Gerudo Desert or Lanayru Desert).