Currents are universally present in the oceans, but they are also found in inland waterways. The direction and velocity of the current, as well as its stability/uniformity, must be estimated by the dive professional in the role of a group leader. Rapid currents, especially when encountered in a limited space, such as in a river, are extremely hazardous, and a diver must undergo specialized training in river diving prior to attempting dives in this venue. For the most part, the direction of a current flowing is readily ascertained by noting a number of clues: the rippling of the surface of the water, drifting or floating of partially submerged objects, the drifting of the boat or other divers. The velocity is not quite as easily estimated, but a simple trick is to drop a floating object in the water at point A, time how long this object takes to cover a given distance to point B, and make the necessary mathematical calculations that give the estimate in kilometers per hour.