Lost places or abandoned places in Ukraine
Ukraine is the place to go if you like to explore abandoned places.
Some of them are more or less famous tourist attractions like e.g. Tarakanov Fortress.
https://www.ukraineinsideout.com/en/castles/tarakanov-fortress.
Here you are probably not going to be alone for a long time because a lot of other people also want to discover this amazing fortress that has almost been taken over by nature. Others are completely unknown like this old manor house, that was used as a school and still contains educational material, and here you can explore on your own without being disturbed.
http://www.ukraineinsideout.com/en/palaces-and-manors/palace-lozhinski
Maybe you have always wanted to be an astronaut (at least as a child) then this place might be something for you.
http://www.ukraineinsideout.com/en/abandoned-places/abandoned-space-camp
But the most famous (and scary or exciting) abandoned place in Ukraine is Chernobyl in the north of Ukraine. Here a nuclear accident at the Chernobyl Nuclear Power Plant the 26th of April 1986 has contaminated the area so people can’t live here anymore. As a result, the town of Chernobyl is now a ghost town.
On your own you can’t go any further than this, but you can book a guided tour from Kyiv that will take you into the abandoned town.
https://www.ukraineinsideout.com/en/abandoned-places/chernobyl-entrance
When you are exploring old buildings, always be careful because no-one is checking if the location is safe. So you have to take care of your own safety. And remember to leave the place in the same condition as when you arrived.