Apart from the beautiful buildings, houses, and shops you can enter and explore the vast amount of detail, loot, and story that is driven into the game, you will also be able to find Caves.
You will find Caves all over the map, each cave will be indicated on the map by the "CAVE ICON". When being in a Cave, your radation level will rise. In this patch it doesn't do damage to you health or makes your food and water run out faster.
The blue liquid you see here is the radiation. Getting closer to it raises your Radiation Level.
There is also the posibilty of loot being in the Cave, as well as fixing up a Generator which will light the whole Cave up.