Please Tell Me How to get the idol?

I have no clue what it is....if u dont want to spoil it please just tell me what it is and not how to get it....
I checked the spoilers and i still do not know what it is
Thanks

At the "bottom" of the lagoon is a large rock. Have a builder "check it out" and see what happens.

If you need hints, there are some in another section of this area. It's best that you check that out - I don't want to 'give it away' but the answer is there.