Gollum at Mordor
 

The Tale of the Years in RotK states that Gollum was captured in Mordor but why did he go there? I know that he was searching for the Ring but what reason would he search in Mordor (only to be captured there).

Gandalf's theory, as explained to Frodo, was that Gollum, being enamoured of malice and having been a Ring-bearer for a considerable amount of time, was 'drawn' there by Sauron, who was actively gathering all creatures of evil to Mordor. The Ring's lasting influence would have left him especially vulnerable to the call. Also, he had heard that the Power in Mordor was rising against the West, and he had some hope to find friends there, and knowledge that would help him regain the Ring.