From Alathra Wiki

A rumbling was felt throughout the world of Alathra... in an instant, the bedrock layer exploded below the surface creating a multitude of new ores and minerals never before seen by the people here! This event was not without consequences however... as a result of the downward expansion of the bedrock layer into the void, the mysterious link between the Overworld and the Nether seems to have been lost... It is not yet known how any of this has happened, or how the link between worlds may eventually be restored, but there are a select few elite individuals determined to find the way.