Hey everyone, whilst searching Wuhu Island into Wikipedia, I found this:
"History
Wuhu Island's history before the Miis colonized it is shrouded in mystery. However, based on the sharp contrast between the ruins on the north side of the island, which appears to have been built by a highly advanced civilization, and the huge, crude, half-destroyed monoliths on the southern side, it would appear that there was a battle over the island. It would seem that the less advanced of the two civilizations were a stone-age, tribal-based people. Their monuments were destroyed very thoroughly, and may have been toppled by the more advanced civilization. The more advanced civilization, which built "The Mysterious Ruins" on the other side of the island, were incredibly advanced stone masons. The city which they build on the north side of the island was built without mortar, but the stones were so well laid, that they didn't need it, and it has stood for thousands of years. They also carved tunnels deep into the volcano, and have tunnels have been found leading from their city down into the underwater caverns beneath the island."

I found this very interesting in that it supports ' the door theory'!
Please leave your thoughts below!

Ok people, I have since found more interesting stuff about the ruins. On Wikia , there is a list of landmarks on Wuhu, and the messages which appear when you fly past them in the island flyover activity on Wii Sports Resort.
"Mysterious Ruins
At Night: "A pit was discovered in the ruins, but everyone is too scared to explore it!"

"Entrance to the Mysterious Ruins
Night: "Sometimes you can hear eerie singing at night..."

I would really reccommend reading the page, so much evidence in there: http://nintendo.wikia.com/wiki/List_of_ ... _landmarks

For original Wuhu article: http://nintendo.wikia.com/wiki/Wuhu_Island
I feel like we're getting close to an answer!

It is really interesting! But didn`t Miis settle on Wuhu in 2008 when the old Wii Fit was released? And didn`t they build in the end of 2008 and in the beginning of 2009?

Wow, very interesting history! I love ancient stuff; thanks for the info. I keep hoping a new version of Wii Fit Plus Plus will take us deeper into the mysteries.