Answer the following question given this paragraph: 

Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. Density is defined as the amount of mass per unit of volume. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense?.


Q: Deep ocean currents are caused by differences in what property at the top and bottom?


A:
density