A Remotely Operated Vehicle (ROV) is an underwater robot that allows the vehicle's operator to remain in a comfortable environment while the ROV performs the work underwater. An umbilical, or tether, carries power and command and control signals to the vehicle and the status and sensory data back to the operators topside.
In the case of ROV technology the answer is quite simple. There is no other practical, safe and economically feasible way to perform deep underwater intervention. ROVs are one of the best tools available to researchers as they can withstand pressure at extreme depths while minimizing the risk to human life and at the cost of valuable resources. ROVs may well be the best tool available to repair non-functioning equipment.