Kaer is an underground, warded city. Dozens, if not hundreds, were built during the Fourth World, in preparation for the Scourge. The science of building kaers was researched by the powerful Theran Empire (located on an island in what is now the Mediterranean Sea) and spread to nearby civilizations (usually in exchange for submission to the Empire).
Kaers were built underground, as Horrors have trouble passing through living Earth. They were also protected by powerful magical wards. Some kaers survived the centuries-long Scourge, many however have fallen to the Horrors (a guesstimate 50% rate of failure may be applicable). Some kaers were breached by powerful Horrors from the outside, others were penetrated by Horrors before they were sealed, a few were destroyed when their ecosystems failed.
The remains of a few kaers can still be found by archaeologists of the Sixth World, although few realize their true purpose and age.