There are several collectibles which take the form of medium-sized black discoball-like orbs, hidden throughout the various levels of Inside. These orbs are also always attached to yellow power cables, which can often be seen from outside of the secret areas where the orbs are hidden. There are fourteen orbs total, the last one near the end of the game is actually attached to a huge board that tracks your progress, and a dot on the board will light up for each of the other thirteen orbs you discover and break open.
Since you can simply load up any area of the game you want, it is really easy to backtrack and pick up additional orbs that you might have missed. Opening and destroying all the collectibles inside the black orbs will open up the final and largest orb, allowing you to break open the last collectible and claim a final secret.
This first orb is located in The Forest, close to Checkpoint 6. After you are chased by the team of dogs, and dive off the cliff into the water, dive down and look to your left to spot a secret path.
If you follow the path it will lead you to a room with the first orb. Rock the Pillar to create a bridge across the gap in the floor, and then destroy the orb.
This second orb is located in the Cornfield, a little after Checkpoint 7. Walk forward once you enter the cornfield, and just keep pressing down on the Action Button until your character stops and grabs on to something. This is a vault door, which leads to an underground bunker with the next orb hidden inside.
Climb into the bunker after opening the door, and then look to your left to spot this orb in a photo development room. You might also have noticed the inactive vault door on the right of the room. Ignore this for now, however, once you have collected all the orbs, this door will become active, and you can unlock it to acquire a secret ending.
This orb can be found at The Farm, just after Checkpoint 9.
Drop down from the top of the farmhouse and into the wagon of dead pigs. Now take the wagon and pull it to your left, next to the building. This will allow you to climb on top of it, and pull a plank off the side of the building so you can drop inside.
Now use the rope to swing to the left and grab the secret orb to destroy it.
The fourth orb can be found near The City, at Checkpoint 14.
When you enter the city, you will have to find a way to the top floor to push a safe off of a plank, and off the side to destroy another group of planks, that opens up a hole in the floor.
After dropping the safe through the floor, climb into the hole, enter the vent in the left-hand corner, and then destroy the secret orb inside.
This orb can be found in The City, beneath Checkpoint 16.
Once you reach the rooftops, make your way to an area where you have to slide down the roof to another building, and then jump across a small gap onto a pipe. Instead of climbing up, like you would to continue the story, climb down, and jump across to a ladder. Now climb down the ladder to find this orb and a large tape player playing some eerie tunes. Take note of the sequence of tunes, as you will need them to find the secret ending.
To find this orb you must travel to The Depths, between Checkpoints 29 and 30.
After you jump over a small ledge, and then smash open a red door with the submarine, you will enter a flooded room featuring a large stairwell that ends with another red door.
Head down and dislodge the crate near the bottom of the area by charging it. Then continue forward down the path into a secret puzzle room. To reach this orb you will need to use the mind controller to put the guy on top of the submarine.
Now hop back into the sub with the guy on top, and carefully lower the sub into the water to launch yourself into the air at the other mind control helmet. The man on top of the sub will reach for it, and grab a hold of it. Then you can make your way to the previous mind control helmet, and use it to control a new character in the background that will allow you to interact with the orb and destroy it.
The seventh orb can be found across from The Beach, between Checkpoints 31 and 32.
After you crash through a wooden patch in the ceiling, turn to the left, and make your way all the way over in the submarine. Now use the charge underwater to launch into the air and hit the pipes twice. Once that has been done, go ahead and exit the sub and make the jump to the new ledge and ladder, climbing up them to discover a secret orb that you can break.
This orb is located in The Mines, close to Checkpoint 34.
To find this orb you will need to make your way through the Mines until you must complete the “20-man puzzle”. First you will need to gather a few followers from the bottom floor, where you first get the helmet. Now use those followers to gain access to the very left by using the minecart and rope to find three additional followers in a cage.
After releasing the caged people, make your way back to the elevator and destroy the patched up wooden wall you spot along the way. Head inside the hole, down the nearby ladder, and take the torch so that you can defend yourself against a group of wolves coming up.
Now make your way to the left of the cave, and keep the wolves at bay until you can destroy the orb, and force them to scatter.
To find this orb you must make your way past The Bridge, around Checkpoint 44.
Move forward from the checkpoint, and enter the water using the right-hand floor panel. Wait for the creature beneath the surface to get close, then jump back to the surface to escape its jaws. Now head to the left and jump in the water, swimming all the way left to outpace the creature and reach the hidden orb ahead, as well as a crate that leads you back to where you came from.
The tenth orb is located in The Flooded Base, near Checkpoint 46.
Swim to the right from the checkpoint to reveal a yellow power cable. From here you will need to swim up, through the building, and into the next area beyond. Now swim down and to the left to find a small vault door that can be opened by rotating the middle piece, and pulling down on the tab. Follow the path the door was blocking to find the orb.
Once you reach the Facility Entrance, near Checkpoint 48, you won’t have far to go to snag this orb.
From the checkpoint, take note of the waterlogged room which hides a pit near the end. You can swim down from here to an underground lake with the orb hidden just out of reach on top of a clockwork machine. If you head back up, you will come to a puzzle that requires you to rotate platforms at the end of a long piece of machinery, which is used to shield you from the light scanner.
Position this piece at the very center of its highest position, and then head back down into the water to find a platform that will allow you to reach the orb.
This orb is located in The Facility, close to Checkpoint 50.
Make your way to the room with the floating bodies, and then solve the puzzle until you are able to get up to the bodies. Now you need to continue the puzzle until you are able to return the yellow box to the room with all the bodies. Use the box to allow you to push the button that will lower a platform down along the right-hand wall.
Now use the box one last time to jump onto the platform, and ride it up to the top to reveal a secret orb.
The second to last orb is located in the Facility Offices, near Checkpoint 54.
During the elevator puzzle, make your way to the bottom of shaft and then bring the elevator to the highest level. Now grab the nearby followers on top, and have them follow you to the same level. Head into the elevator, and close the door quickly to cut the followers into two different groups. Take the elevator down, forcing the second group of followers to climb on top of the elevator cart.
Now take the elevator back up, and exit it to the right. Use Group 1 of your followers to boost you to the yellow wire, then climb to the top. You should now be in the attic, and you can use Group 2 to boost to the orb nearby.
The final orb is located in The Facility right after Checkpoint 58.
Once you have unlocked all thirteen of the previous orbs, the final secret will be reachable. From Checkpoint 58, look forward to spot a ladder. Jump across and grab onto the ladder, climbing it to the top to discover a large room with a giant sphere. Head inside the sphere and move the object inside to deactivate the lights, and destroy the last orb.
Now walk back outside of the sphere to unlock the Last One achievement.
Once you have destroyed all fourteen orbs, you may notice that all the lights in the background, which showcased the amount of orbs found and destroyed, have all gone out, save for one, the second one. This is a clue, that when followed will lead you back to the second secret orb, where you can discover Inside’s secret ending hidden behind one last puzzle.