I never work with heightfields, but there are some additional nodes that can help, like the clip vertical and adjust vertical. Maybe you can lower it using those. You could also (I guess) use the displacement to scalar node, add a smooth step, and some constants (the heights you need to show), and feed that into a diplacement shader, which will shape your terrain. Or you could place the heightfield on a second planet, or sphere, same size, and add a surface layer or displacement shader with a constant (offset) to lower the main surface, so the heightfield will sink into the main planet.