You'd fall through clouds until pressure crushed you into a supercritical fluid. There's no clear boundary between atmosphere and interior. Deep inside, hydrogen becomes metallic — a state only possible under unimaginable pressure.

The exact chemistry is unknown, but likely compounds of sulfur, phosphorus, or organic molecules churned up from deeper layers and turned red by solar UV radiation. The spot has been observed continuously since 1830, and may have been seen as early as 1665.

Europa's global ocean contains more water than Earth. Hydrothermal vents on the seafloor could provide energy for microbes, similar to Earth's deep-ocean ecosystems. NASA's Europa Clipper mission (arriving 2030) will study its habitability.

Yes, with heavy shielding. NASA's Juno spacecraft uses a "radiation vault" to protect electronics. Future Europa missions will need similar protection. The radiation is so intense it can damage spacecraft electronics in hours without proper shielding.

Concepts for a Europa lander to drill through ice and sample the ocean directly are being studied, though that's decades away. JUICE will also study Ganymede in detail, the only moon with its own magnetic field.