Because it has a Cave that is a Distorted and Messed Up and somehow it shows up after the Elite 4 :O spooky O:
Look at your map and you'll find an area that wasn't there before that leads to the fourth lake of Sinnoh that was kept secret. (It's in the description)
Turnback Cave. It is known as the "Fourth Lake of Sinnoh". It is only unlocked when you have completed the game and beaten the Champion.
The "secret fourth lake" of Pokemon Pearl is called, Sendoff Spring, it can be found east of Route 214. Sendoff Spring is an endgame area that is only accessible after beating the Elite Four and getting the National Pokedex.
Pokémon Diamond, Pearl, and PlatinumThe lake guardians each live in the cavern at the center of one of Sinnoh's three commonly known lakes: Uxie resides in Acuity Cavern of Lake Acuity, Mesprit resides in Verity Cavern of Lake Verity, and Azelf resides Valor Cavern of Lake Valor. A secret fourth lake exists, but instead houses Turnback Cave, which contains a portal to the Distortion World and is thus often visited by Giratina.
located in the region of sinnoh
Lake Erie is the fourth largest great lake, with Lake HUron being the third
Lake Valor, Lake Acuity and Lake Verity ... Simple Lake Verity, Lake Valor and Lake Acuity, also for advice there is a fourth lake which is kept secret and is not known until you get the national pokedex. The fourth lake is called spring path. Also to get the national pokedex you need to know every Pokemon in the sinnoh pokedex (You can include Manaphy but you don't need his data for the national pokedex). To get Manaphy you must get the manaphy mission on Pokemon ranger in the ranger net. Also because I am a helpful guy and a Pokemon fan that couldn't imange a world without Pokemon I am allowing more information. Get : Garatina from Spring path, He is level 70 Uxie from Lake Acuity, He is level 50 Mespirit from Lake Verity, She is Level 50 Azelf from Lake Verity, He is also Level 50. Last bit of advice Mespirit flys out of the cave when you go to Lake Verity so when you go to her she will fly away and you will need the Poketech icon ''Marking Map'' so that you know what part of Sinnoh she is hiding. If you use Fly by the way Mespirit will fly somewhere else in Sinnoh, So keep your Honchkrow's fly for battling Mespirit. And when you find Mespirit beware, She will flee if you don't catch her as soon as you find her so you must know Mean Look or Block.
lake verity is only in the sinnoh region. not kanto
The Secret of Hidden Lake was created in 2006.
if you beat the elite 4 and go 2 the island, a secret Sinnoh Lake will appear. inside the cave, if you go the right way, is Giritina. It is lvl 70, so be prepared.
Emrit' or 'Mesprit' is legendary Pokémon of the Psychic-type. It's one of the three 'Guardians of the Lake' of the Sinnoh region and appeared onward the fourth generation of Pokémon's. In the Diamond version of the games it can be found roaming around the Sinnoh region, where can be trapped if defeated.
The duration of The Secret of Convict Lake is 1.38 hours.