Iceberg Homes are pretty much what you imagine them to be. Houses that are bigger below the ground than they actually appear to be. It is a growing trend in architecture (especially for homes belonging to billionaires) where they build a massive underground structure, but have lots of room to store all their things... And for entertaining guests / having parties.
Basically it is just another way for the rich to show off their wealth.
There's only one major flaw: They tend to get flooded easily. Definitely need flood insurance.