This is not just about urban exploration, but real archaeology.
In the year 2013 i participated 2 weeks as a volunteer in the excavation works at the Bosnian pyramids. In the year 2006 discovered the businessman Semir Osmanagic, that at least 2 of the mountains near the small city of Visoko are pyramids, erected by a yet unknown culture. He called them the Pyramid of the Sun und the Pyramid of the Moon; there is also a Pyramid of Love and a Pyramid of the Bosnian Dragon, but maybe the last ones are just natural mountains, this is not really clear yet. Moreover there is an extended labyrinth of underground tunnels near the pyramids, which is an important part of the excavation works. It has been (probably millenias ago) completely filled up with gravel, in which you can find artifacts of stone.
The Pyramid of the Sun is hidden under a surface layer, which is dated by geologists of an age of at least 10.000 years. The pyramid itself is coated by a layer of ancient concrete; georadar measures showed tunnels below that coating, but nobody found an entry yet.
The Pyramid of the Moon has no concrete coating and consists of layers of stonework.

The concrete layer at the Pyramid of the Sun.