No, light does not penetrate into the deep underground of Earth, such as the Underdark, unless there are specific sources of light present. The Underdark is typically depicted as a vast, subterranean network of caves, tunnels, and caverns found in fantasy settings.
In reality, once you go deep enough underground, light from the surface cannot reach those depths. The reason is that the Earth's crust is opaque, and it blocks the transmission of light. Additionally, the further down you go, the less likely it is for any natural light to reach those depths due to scattering and absorption by the Earth's materials.
However, it's important to note that in a fictional or fantasy setting like the Underdark, the rules of reality can be altered. Writers and creators often introduce sources of light, such as bioluminescent organisms, magical artifacts, or underground fires, to illuminate these deep subterranean environments. These artificial sources of light allow for visibility and the creation of an atmospheric setting within the narrative.