The Lost and Found is a location that first appears in the episode of the same name. It is the Krusty Krab's lost and found, where all objects (or people) left in the restaurant are put until they are reclaimed. In order to enter it, a person must go through a trapdoor located in the kitchen.
The lost and found is a large storage area under the Krusty Krab's kitchen. It has many shelves, boxes, and barrels filled with all sorts of objects. There is also a mine with miners mining gems and a world filled with extinct marine reptiles from the Mesozoic era.
Over 60 years ago, Ginger and three other kids were put in the lost and found and never claimed. They survived by eating the food people had lost, like gum and mints.
Many decades later, a little child enters the Krusty Krab, crying about how he lost his Teddy fish. Mr. Krabs sends SpongeBob to the lost and found to retrieve it. While inside, he encounters Ginger and the other lost kids, who celebrate by having a welcoming feast (made entirely of candy).
During the feast, SpongeBob notices that Ginger has the teddy fish. After tricking her into giving him the toy, Ginger and the other kids chase SpongeBob around the lost and found on lost toys, toilets, pets, through a mine and a world with dinosaurs.
In another part of the lost and found, Squidward (who was ordered by Mr. Krabs to go find SpongeBob and the teddy fish) opens a latch filled with grease, thinking it is an emergency exit. The grease then starts to flood the room.
SpongeBob, who is cornered by the lost kids, refuses to give them the teddy fish. The area then starts flooding with grease, causing the lost kids to climb on each other's shoulders to try and escape. SpongeBob eventually uses a Tylosaurus from the lost world to open the hatch, allowing him, the lost kids and the Tylosaurus to escape the lost and found.
At the end of the episode, Squidward (who is blowing on a kazoo he found) panics as he is swept away in a river located in the lost world.
Objects seen in the location
The lost and found has tens of thousands of objects in it. These include:
Lost mine (complete with miners, gems, mine carts and a track for the mine carts)
Lost world (complete with prehistoric reptiles, including a Tylosaurus - which was ultimately claimed by a child - and a Tanystropheus)
Lost dead ends
Jacks and marbles
Due to Ginger saying that she and the other lost kids were put in the lost and found over 60 years ago, it can be assumed that the lost and found contains at least some objects from at least 60 years before the events of "Lost and Found" occurred.
This place was mentioned in a tweet by Patrick Star on February 22, 2018, where he questioned if the key to happiness is in the Krusty Krab Lost and Found.