Venus is the canonical dead planet — 470°C surface, crushing atmosphere, no magnetic field. Garcia, Barnes, Driscoll, Meadows, and Gialluca model its thermal history and find that it's more alive than it looks. At least one Earth ocean's worth of water likely remains trapped in the mantle. The planet is probably still volcanically active. And in 88% of plausible thermal histories, Venus once had a magnetic field.
The water result is the most surprising. The surface is desiccated — atmospheric water vapor is negligible. But the mantle is a different reservoir. Water can dissolve into silicate minerals at high pressure, and the amounts that survive mantle convection over 4.5 billion years depend on the thermal evolution, not the surface conditions. The surface says “dead.” The interior says “wet.”
The magnetic field result completes the picture. Venus has no present-day global magnetic field, which is usually interpreted as evidence that its core has solidified or its dynamo never operated. But the thermal models show that a dynamo was probable for most of Venus's history — it died relatively recently (geologically speaking) as the core cooled past the dynamo threshold.
What this changes is the inference from surface to interior. Venus's hellish surface is real, but it tells you about the atmosphere, not the planet. The mantle water, the volcanic activity, the historical magnetic field — these are properties of the deep interior that the surface conceals rather than reveals. Venus is not Earth's opposite. It's Earth's sibling in disguise.