Portals

The Portals are buildings that connect to an alien city, seemingly existing within in the buildings. Each connects to the same city, so it is likely the city exists on another planet or another Dimension. It is home to a myriad of alien from different races. Due to the superhuman abilities of some of these beings and the desire to keep their presence a secret, all crimes committed by aliens are deal with by the SSSP

Summary
The city exists, most likely in another dimension accessed in buildings, aptly called Portals. Only Authurized individuals may visit there such as SSSP members. The fact that certain aliens are able to commit crimes on Earth and remain mostly anonymous implies that there may be other ways in and out of the city besides the Portals which are monitored.

Inhabitants
There are numerous alien races living in the city, some are humanoid others not so much, what is known is that they all coexist and socialize. Of the species seen many are undocumented but several bear striking similarities to other aliens seen in the Ultra series.