https://tibiamaps.io/map#33310,31996,7:2 not Not really sure if its consider a building but it was for sure built by either dwarfs or humans the hole that let you get to the steamboat in cormaya goes from 0 to -8
and the Cormaya goes from 0 to -8
The construction the other person is mentioning is this https://tibiamaps.io/map#33512,31333,8:2 wich which is a stair that takes you a temple said stair only goes down from -1 to -8 if you count the hole that let you access that stair then is 0 to -8 but risking to offend i dont I don't think a hole can be count as a building not even a structure
there structure.
There are also several buildings that are -7 floors deep in the ground one example could be the tombs of the pharaos in ankrahmun pharaohs in Ankrahmun like this one https://tibiamaps.io/map#33252,32838,7:1
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Actually making my investigation notice that 2 of the tombs of the pharaos pharaohs goes from 0 to -8 so those could be consider the buildings that can access the lowest level floor from ground but you have to use a coal basin and mystic flame with a scarab coin to gain full access and this flame leaves you in the same floor so im I'm not sure if this will have any effect in the answer since you said no levers or portals Proof: https://imgur.com/a/UrDdbGu ------- on the other side https://imgur.com/a/cGtMUlM
this This is the place that requieres requires the scarab coin https://tibiamaps.io/map#33240,32853,13:2