Properties of water change dramatically at high temperatures and high pressures near and/or above the critical point (T_c = 374 °C, P_c = 22.1 MPa). The dielectric constant, for example, decreases from 78 at 25 °C and 0.1 MPa to 6 at the critical point, the value of which is comparable to that of 1-dodecanol. As fascinating characteristics of soft matter rely on unique properties of ambient liquid water, the change should have significant impacts on soft matter. However, our knowledge of soft matter under such extreme conditions is virtually nonexistent. In this article, properties of colloidal dispersions in water at high temperatures and high pressures are described. Implications of the findings for geological processes in deep-subsurface are also discussed.