Landscape
The Under Depths is a Subterranean space beneath the The Sylvan Glade. It's filled with various tunnels and caverns that houses a variety of individuals. Concealing those from the light, those who thrive in the dark can live.
Climate
Depending on where you are in the Under Depths it does vary in climate. Some areas are humid and hot due to hydrothermal vents that release heat from the planet's core. Other areas are going to be frozen solid and extremely cold with various icicles forming along the roof.
Natural Resources
Although often considered inhabitable there are areas filled with a variety of natural resources. Ground water, rivers, and under ground lakes are concealed within the space to allow vegetation to flourish despite the lack of light. This has allowed a variety of agriculture to accumulate.
Seasons
Unlike above ground, the Under Depths does not experience seasons the same way or if at all. Areas that are already warm will see higher temperatures during the summer and cooler temperatures during the winter. Otherwise, the changes are hardly noticeable.
Weather
The Under Depths does not experience weather the same way that those above will. They will however, see bits of rain fall. Water will seep through the upper levels and create this precipitation down below. In areas that are completely open and exposing the Under Depths will experience normal weather activity such as sunlight, rain, snow, and other means or precipitation.
Animals
Here in the Under Depths much like it's sister the Underdark, there will be a variety of creatures that will live here. Cloakers, beholders, and more are amongst those to be mindful of when out and about. However, the land is also filled with a variety of bats, moles, spiders, and other burrowing mammals
Plants
Despite being mostly underground, a variety of plant life does thrive within this region. A specific type of orchid can live within the subterranean environment. Other plants include potatoes, truffles, lichens, moss. The Under Depths are mostly home to a variety of fungi which are often harvested and consumed.