Unlike previous crises, the Lemuria crisis mission is repeatable. It may be accessed via the submarine on Monster Island once you accept the first mission (available at level 31). There was a period where level 31's were unable to actually enter the crisis zone, but this appears to have been resolved. The crisis mission is in four parts. The first three phases are soloable - the final phase is the most difficult boss fight you've encountered to date and is not actually necessary to unlock access to Lemuria.