I read this book a couple of years ago and can't remember the name or the author (also not sure if it was just one book or series - if it was more than one book then it would be 2 or at most 3 books) - anyway the plot is about a group of people finding an alien ship or machine that contains these doorways into alternate or paralell worlds. If I recall correctly I think there were a couple of these ships/machines found and one was in South America although that was later in the story.The aliens are nowhere to be found so the people start exploring the ships and find the doorways. After going thru one of the doors they find that they can't get back to their original timeline or world and they keep trying to find out how to get back. Also they find artifacts and machines left by the misssing aliens who created these ships/machines and try to find out about them. One part that I remember - After the world at large has found out about the ships/machines - one way they use the doorways are as prisons by sending criminals thru the doors and sending supplies thru and just let these criminals sort things out amongst themselves. I know this is really vague. I usually download most of the books I read and then delete them from my e-reader. Any help is appreciated.