Board Thread:Suggestions forum/@comment-29954640-20170608225244

"And the saps and cavernous places beneath the surfaces of the earth are full yet of the of the dark spirits that were prisoned that day when Melko was taken, and yet many are the ways whereby they find the outer world from time to time – from fissures where they shriek with the voices of the tide on rocky coasts, down dark water-ways that wind unseen for many leagues, or out of the blue arches where the glaciers of Melko find their end." (From The Book of Lost Tales 1, The Chaining of Melko)

This quote mostly explains it, but maybe Utumno invasions could sometimes spawn in places like that. 