Understanding how water behaves inside large rocky planets (super-Earths) is important for studying their structure, water storage, and potential to support life. Traditional models treat water and rock as separate layers, ignoring chemical reactions between water and iron that could form compounds like FeOOH. In early planetary history, these planets likely had deep oceans of molten rock (magma oceans). As they cooled, leftover melt became rich in iron and hydrogen, possibly forming a dense molten layer (basal magma ocean) near the planet’s core. To investigate this, we used powerful lasers to compress FeOOH to extremely high pressures—over 800 GPa—and tracked how it melts and how iron behaves inside. Using X-ray techniques, we found that FeOOH melts at 95 GPa and its melt contains mostly low-spin iron above 265 GPa, which is denser and more stable at great depths. We combined these results with computer simulations to model planet interiors. If iron reacts with water to form FeOOH, a super-Earth could be smaller and denser than previously thought—by up to 30% in radius and 220% in density. These differences are large enough to be observed with current space telescopes, offering a new way to study exoplanet formation and water storage.