Underwater logging is the process of logging trees from underwater forests. When artificial reservoirs and dams are built, large areas of forest are often inundated; although the trees die, the wood is often preserved. The trees can then be felled using special underwater machinery and floated up to the surface. One such machine is the sawfish harvester. There is an ongoing debate to determine whether or not underwater logging is a sustainable practice and if it is more environmentally sustainable than traditional logging.

Underwater logging has been introduced in select locations around the world, including Ghana's Lake Volta, the largest reservoir by surface area in the world.

A related form of logging consists of salvaging logs which loggers have abandoned after they became waterlogged and sank. This activity can be quite profitable, since the prime "targets" are decades-old trees of a size and species difficult or impossible to find in their natural habitat.

Rivers were a main method of transportation in the logging industry in the 19th and early 20th centuries in the United States. In the spring, logs were floated down waterways, especially those surrounding the Great Lakes and Maine, to transport them to mills downriver. Logs with a higher density than the density of water would sink. Other logs would get caught in jams, sloughs, or floods, and become lodged in the riverbed. Such logs were often known as "sinkers" or "deadheads." Loggers attempted to reduce the number of logs which remained in the river in order to maximize profits, but some losses were inevitable. Logs with legible log marks were sometimes returned to their owners.

Underwater logs are safe from many of the forces which cause decomposition, including fungi. Log salvage operations began in the early 20th century across the United States. John Cayford and Ronald Scott's book Underwater Logging describes the process and prospects for retrieving sunken wood from American waterways, known as salvage logging.

Salvage logging differs from underwater logging. Salvage logging recovers full-sized logs that were lost during past logging expeditions. Underwater logging uses new technology to cut down drowned trees that have been lost due to rising water levels or artificial reservoirs.

One method of unearthing sunken trees is by sending a remote controlled vehicle, like a sawfish harvester, underwater to fell the trees. The vehicle is controlled by a cable that sends electricity and control inputs to the vessel which sends back a video feed for the operator. The operator sends inputs from a control panel on a barge. When a tree is found the Sawfish attaches and inflates a flotation device to it so that after the tree is cut it immediately rises to the surface for extraction from the water.

Attaching buoys is one of the main processes by which underwater logs are salvaged from the bottom of lakes and rivers. First, a scuba-diver must locate the sunken logs in the water, searching from about three feet from the bottom of the lake or river. Then, a buoy is placed around the log about three feet from its back. From there, a boat uses a gaff hook to catch the buoys and pulls the log close enough to the boat where the crew is able to tie the logs close to the side of the boat. This process repeats itself until the boat is filled to its capacity, after which the expedition is completed, and crew must return to base before harvesting any additional logs.