The Uuserk Marshes are an inhospitable region of swamps and quagmires in Zhalfir. It was here that the tomb-city of Aku was built, far away from civilization to discourage grave-robbers.
The trails leading to the marshes were closed shortly after the Mirage War broke out, when the Femeref scout Ekemet and her troupe mysteriously died in the swamp. Soon afterwards, the city of Aku closed its gates, until the final magical duel in the swamp where Kaervek was defeated.
The Uuserk has been phased out alongside the rest of northwestern Jamuraa since the planeswalker Teferi wanted to spare it from the horrors of the Phyrexian Invasion. It is rumored to phase in again in the Future Sight storyline.