In Platinum, the player and Barry instead come here to catch the Legendary Pokémon of the lake, only to meet Cyrus for the first time. They will then leave when Barry realizes that they don't have any Poké Balls. The starter Pokémon will instead be obtained on Route 201.
About halfway through the story, Mars of Team Galactic raids the lake and captures Mesprit.
In The Needs of the Three!, it was explained that Lake Verity, and indeed all three of Sinnoh's lakes, are the entrances to another world where the lake guardians reside, and they do not actually live in the lakes themselves. Later, a pink whirlpool appeared in the lake due to Pokémon Hunter J dropping the Galactic Bomb into Lake Valor. From this portal emerged Mesprit, who quickly teleported away to Lake Valor. This was witnessed by Professor Rowan and his assistants.
In the manga
Lake Verity in Pokémon Adventures
Lake Verity in Pokémon Diamond and Pearl Adventure!
In the Pokémon Adventures manga
When he was little, Diamond got lost by the lake while trying to find a Red Gyarados with Pearl. He suddenly noticed a mysterious glow over the lake and passed out, later to be found by Pearl and his father, Palmer.
After the battle between the player and the wild Starly in Pokémon Diamond and Pearl, a square of tall grass can be seen to have disappeared when the player and his/her friend walk out of the tall grass. It is where the briefcase which was taken away by Dawn/Lucas was originally. The square remains in Pokémon Platinum even though the same event did not occur in the game.