While the world map may state different countries that you never knew existed all over the world, there are still some areas that have not been mapped yet. One area is the Amazon. The Amazon may appear on the map but some areas of the Amazon has never been explored because of superstitions and religious reasons.
There may be dangers there that are not worth checking out at this point in time. There are also a lot of areas in the ocean that have not been mapped yet simply because they are hard to get into. Even with the vast information about the ocean floor, 96% of the ocean floor is still not mapped yet.
Mainly, the ocean bed. But think of the difficulties: as ice flows melt and coast lines change due to erosion, new land emerges and existing land may be submerged, making any map easily outdated. However, there are land areas that haven't been charted because they are wilderness or contain tribes who have rights not to be disturbed.
there are areas of Chile, Russia, Brazil, the sub-tropical forests of Myanmar (largest tiger preserve), Madagascar, Namibia, Siberia, Greenland, New Zealand, Australia, Vietnam. So none in USA or Europe. However, you say 'mapped' and even within well-visited countries there are areas that haven't been mapped, often because they aren't a good advertisement for the country. So you will find certain cities fully mapped while the adjoining shanty towns are not.
There are a lot of areas in the world that haven’t been completely mapped. One could say that 70% of the planet is still unmapped. Most mapping of the present day has been done with the help of satellite imagery and hence inaccuracy is very likely possible.
The Amazon : The simplest of them would be the Amazon. Much of the Amazon hasn’t been visited by feet. There are various reasons for this. The Amazon cannot be navigated after a certain point. Local superstitions also hamper mapping of the Amazons. There are also many uncontacted indigenous tribes living in the Amazon and their preservation means that they haven’t officially had any modern human contact and still exist in the stone ages. Helicopter exploration is a popular way but the initial 400 km of the Amazon cannot be explored.
Mountainous borders of South America : The borders between Peru-Brazil, Peru-Bolivia, and Chile-Argentina are non-existent in many places. The height and rough geography of the Andes haven’t allowed human exploration of the Andes.
Patagonian Ice Fields: South America is at it again. The Patagonian Icefields have pockets of tourist areas but most of them still remain unexplored and untouched by human feet. North Patagonian Ice Fields are one of the least explored areas of the planet.
The Ocean Floor : Probably the least explored area of the planet is the ocean floor. Only 4% of the ocean floor is mapped and 96% still remains to be explored. Most of this is due to extreme physical conditions under the ocean. Methods are being devised to explore it but they are to no avail.
There are other unexplored lands which I haven’t included such as Greenland, Antarctica etc but these lands have at least been seen at very low altitudes through helicopters.