The Multisensor is a tool that the player acquires early-on in the storyline of Endless Ocean: Blue World. It is used to detect treasures and what materials they are made out of, functioning like a metal detector, except that it can detect five categories of substance: wood, stone, metal, high-density materials, and items that fit into none of these categories.
Items that are made out of wood include things like paintings or books, in which the paper is picked up by the Multisensor, but it also picks up the wood used to make boxes, crates, and even chests used to store treasures. Sometimes these wooden items are simply empty Boxes, which don't have much use, but if a box salvaged also had metal or stone components, it is likely that it is not empty at all.
Stone items include literal rocks (like normal Stones, Marble, and the like), but gemstones also count as stone, like the ones in precious jewelry or jewels in boxes of treasure. Fossils are also categorised as stone by the Multisensor, like Dinosaur Eggs and Petrified Wood. Coral and Seashells are picked up as stone as well.
Metal items include the obvious: precious metals (like ingots of Gold, Silver, or even the legendary metal Orichalcum), jewelry, and coins. However, some paintings may have used paint that contained metals, so some paintings include a metal component as well.
Items that contain high-density materials are most often valuable metals or treasures, but pieces of pottery or glassware (like Octopus Pots or bottled liquids) are sensed as high-density as well.
Any item that the multisensor scans that does not contain any metal, stone, wood, or high-density components is most often a man-made item, like a Baseball or a Glass Marble. These items are usually not as valuable as other items, on average, so most players don't bother salvaging them at all, but some are rather valuable, like Black Pearls.
The first time the player is tasked with using the Multisensor, they detect a lump of Lapis Lazuli, which seems out of place considering the environment. This leads to the player detecting more lapis lazuli items, which in turn leads to the player and the rest of L&L Diving Service discovering the ruins of Triton Village.