Significance Astrophysical observations have shown that Neptune-like planets are common in our galaxy (sub-Neptunes). Some of these exoplanets are believed to be covered with a thick H 2 O layer (100 to 1,000 km in thickness) above the rocky mantle (“waterworlds”). In order to understand the inner workings of the water-rich planets, it is important to understand the state of matter incorporating ice- and rock-forming elements at high-pressure and high-temperature conditions. Here, we report experimental evidence that silica and water have significant mutual solubility at high pressure and high temperature, forming new phases containing substantial amounts of both H and Si in oxide forms. Therefore, the boundary between rock and ice layers may be “fuzzy” at the deep interiors of water-rich planets.