In my 'Bulgaria section' you can find many more places to travel to!
At 111 kilometres from Sofia and 9 from Lukovit is the Prohodna cave. With two wide eyes in the ceiling, many refer to the place as 'The Eyes of God' or 'Oknata'. Parking is on the field outside the large entrance. There's no parking or entry fees. The cave is easily accessible and a popular destination for people from Sofia, Pleven, Vratsa and even places farther away. As you come from the parking, you enter the small entrance, which is 35 metres tall. You're welcomed into the 262 meter long tunnel. It's the longest cave passage in the country. It isn't quite 'into the ground' so this doesn't mean that this is the longest cave in Bulgaria. Literally, Prohodna means 'passage cave'. So, when you get to the other side, you find the large entrance, which is 45 metres tall. In addition, it's Bulgaria's largest cave arch.
Evidence of human presence was found, people used the cave starting anywhere from 3500 B.C. to 1700 B.C., during the New Stone Age and Bronze Age. The cave is quite long, the two holes in the ceiling light up the cave which would otherwise be a bit dark in the middle. The best moment to visit is at sunset, when the last beams peek through the eyes and light up the cave with golden light. I did see pictures of the moon, captured through the eyes, yet the cave is not artificially lit so you'd have a hard time getting around when it's dark. The eyes are not man made, they were created by millions of years of erosion.
You can walk to the other end of the cave and it is said that there's a path to the Cave house, officially 'Peter Tranteev National Speleological Society'. I didn't find the path and went back up to the parking. I drove to this Cave house, which Bulgarians call 'Peshteren dom'. If you get back to the main road, turn right and directly left afer the wooden tourist information building. At the end of the road is a small parking. Enter the building and walk two floors down. The terrace has a good view over the valley. The buildings down next to the railroad, just in case you wonder, are part of a psychiatric hospital.
Where to stay
The Cave house offers accommodation - yet it is best to book in advance. In the article The Lukovit area, I list several other options for accommodation. I actually recommend staying in Lukovit, where you can have dinner in Restaurant Rodina and sleep in the Diplomat Plaza or Diplomat Park Hotel.
What else to see
If you check Google Maps, right near the village Karlukovo is 'Temnata Dupka'. This is a much longer cave, I haven't been but will certainly provide more information as soon as I have the chance!
Prohodna cave can be reached by train but it takes forever to get there from Sofia. Best is just to have your own car, so you can visit everything in the Svoge area, Lakatnik and the Lukovit area. And why not visit Pleven and Lovech as well! Then, a sight that's not very well known, the Strupetski monastery 30 kilomeres west of the Prohodna cave. If you choose this route, it's nice to drive to Sofia through the Iskar gorge with the Cherepishki monastery, Sedmite prestola monastery, Lakatnik, Tserovo and Svoge.
On the left you can see a video of the cave. I visited first in November 2016 and two times in 2017. Let me know if you have any questions or need help planning your trip!