The Key is one of 201 treasures in Pikmin 2. In order to obtain it, Olimar must go to Sublevel 5 in the Citadel of Spiders located in Perplexing Pool. Use your yellows to defeat the Beady Long Legs and get The Key, worth one hundred Pokos. It unlocks challenge mode. In challenge mode, Olimar must find the key on each sublevel to open up holes and eventually escape the challenge. It is the 196th treasure on the List of Pikmin 2 treasures.
Captain Olimar's Journal
"This item came from the personal treasure hoard of a massive Pikmin-devouring spider. The whole cave was crawling with all kinds of fearsome creatures. I decided to name that eerie cave the Citadel of Spiders. The ship can't stand bugs... Whenever I ask it to store a specimen for salvage, it threatens to stage a mutiny."
"This shape...I have encountered it some where before... No, I must be mistaken.. Yes, I am mistaken. Thoughts like these strike all who see this cabalistic form. You can feel its immense power!"
"Sweet victory! That giant insect's body crumbled into dust!"
"I am quite relieved to know I won't have to store such an enormous bug."
"...Analyzing soil samples. ... ... ... ...Anomoly detected. What's that glittery widget?"
"This object seems familiar. I will combine it with the president's parellel-dimension controller."
"ALERT!!! Sensors indicate a massive dimensional shift at a distant location."
"I'm receiving a message from a far-flung dimension. Displaying the message now..."
""Challenge Mode can now be accessed from the title screen.""
"It appears to be total gibberish. Hopefully the message has reached its intended recipient."
"I will name this apparently worthless device the Key."