1. Information on http://www.metropolitan.bg.
2. Information on http://www.geostabil-bg.com/.
3. M. Kowalów, ETS Europe Tower Sofia Bulgaria, Geotechnical Report, ECE Projektmanagement G.m.b.H. & Co. KG, 2008.
4. M. Kowalów, ETS Europe Tower Sofia Bulgaria, Geotechnical Piles-Raft Foundation Design”, ECE Projektmanagement G.m.b.H. & Co. KG, 2008.
5. S. Shanov, A. Mitev, Geological and tectonic structure below the site of Balkan Center, Sofia based on geoelectrical characteristics and electrotomographic cross-sections of the engineering geological layers, ETS Europe Tower Sofia, Bulgaria, Geophisical Report, ECE Projektmanagement G.m.b.H. & Co. KG, 2008.