The Dark Souls series has always had an air of mystery, from its vague lore to its many Illusory Walls. Dark Souls 3 is no different, and there are four secret areas that you can totally miss if you aren't exploring thoroughly. There's a whole host of bosses and great items that you'll never find if you don't venture to these hidden locations, so you'll definitely want to give them a go. Fortunately, they're not so obtuse if you know where to look, so read on and all will be revealed.
The first hidden area is found right by the High Lord Wolnir bonfire in the Catacombs of Carthus. After the fight (or before, if you really want to), head to the rickety bridge right by the boss room. From Wolnir's side, slice the ropes to make the bridge collapse - turning your side into a makeshift ladder. Climb down and follow the path (there's a demon sub-boss down here) to reach Smouldering Lake.
Consumed King's Garden
This area isn't as tough to find, but you'll have to defeat the Dancer of the Boreal Valley first. From her bonfire, climb the ladder to the rafters and take a left. Along this walkway is a tough mace-wielding knight guarding a lift. Take him out and ride the lift down to reach the Consumed King's Garden.
Reaching the Untended Graves is dependent upon you first reaching the Consumed King's Garden and then defeating Oceiros, the Consumed King. In the tomb beyond, you'll find many items. When you reach the chest at the end of the passage, attack the wall behind it. This Illusory Wall will disappear, revealing the hidden area.
This also requires you to best Oceiros. After the fight, enter the tomb and pick up the Path of the Dragon gesture from the pedestal inside. Now warp to the Ithiryll Dungeon and head all the way down to the bottom floor - the room where the large group of Dungeon Keepers march around, right near the path to the Profaned Capital. Head through the doorway opposite them (this path leads to a lift shortcut you may have opened) and stop at the red mat along the cliff: this is the spot where you might have previously picked up the Dragon Torso Stone. Stand on the mat and use the Path of the Dragon gesture to meditate. After a moment, you'll be teleported to the fourth and final secret location.