Our ability to study the deep ocean is limited because light only reaches down abut 100m and we can see less than 50m underwater. It is an instrument used to locate objects underwater by reflecting sound waves. They lowered a line with a lead weight attached into the water and counted the the number of lengths of line hauled back onboard after the weight hit the bottom. Pressure is extremely high in the deep parts of the ocean. Seawater is corrosive so instruments must be kept from getting wet.
Water absorbs light and other electromagnetic radiation.
We must tow sonars back and forth across the ocean to map the sea floor. Areas the size of Texas have never been surveyed. It doesn't use light and it doesn't use sonar. Find out more about each of the factors listed above as reasons why we know less than we would like about the floor of the ocean.