The ice has less density than liquid water, so it floats on the water surface. Therefore during the winter all the lakes and rivers are frozen only at a top. Deeper layers of water are cold, but still liquid and it allows fish and other organisms to survive. Moreover, the ice cover provides temperature isolation so liquid water beneath the ice has more or less constant temperature (around 4 dgr Celsius)
he outer core of the Earth is a liquid layer about 2,260 kilometers thick. It is made of iron and nickel. This is above the Earth's solid inner core and below the mantle. Its outer boundary is 2,890 km (1,800 mi) beneath the Earth's surface.