Living under the surface of Mercury or Venus poses significant challenges, and it is currently beyond our technological capabilities. Both planets have extreme conditions that make human habitation difficult, but Venus is especially inhospitable due to its thick atmosphere and scorching temperatures.
Mercury: While Mercury's surface is extremely hot during the day and freezing cold at night, it does not have a substantial atmosphere to shield from solar radiation. However, living underground on Mercury would not be a feasible option either. The planet has a thin and fragile crust, and any underground habitats would be exposed to extreme temperature variations and potential seismic activity caused by the planet's contracting and expanding during its day-night cycle.
Venus: Venus has an incredibly thick and toxic atmosphere composed mainly of carbon dioxide and clouds of sulfuric acid. Its surface temperatures can exceed 860 degrees Fahrenheit (460 degrees Celsius), which is hotter than the surface of Mercury despite being farther from the Sun. Additionally, Venus' atmospheric pressure is about 92 times that of Earth, which is equivalent to being submerged deep in the ocean.
Living under the surface of Venus is not practical due to the immense pressure and temperatures. Even if we could somehow create a protective habitat, the challenge of transporting resources and maintaining life support systems under such extreme conditions would be overwhelming.
While it is theoretically possible to create advanced habitats with the technology of the distant future, the current conditions on Mercury and Venus make it incredibly challenging and risky for human habitation. Instead, efforts are focused on exploring and understanding these planets using robotic missions and orbiters to study their geology, atmosphere, and surface conditions.