Mentioned in Eric, this continent sank into the sea over a period of 30 years, parodying Atlantis. Apparently the most embarrassing continental collapse ever, as the population soon adapted to having to wade....
It was abandoned - gradually - by hordes of old men fleeing in small boats, eager to reach new islands where being the possessors of genuinely occult knowledge might well be a way of getting girls.
Ezrolith Churn was known to be writing a seven-volume treatise on Kuian rain-making rituals. These rituals apparently only ever worked on Ku. A question emerges: How could an entire continent take 30 years to sink? If it sank on account of non-stop, high-power rain, it could take just such an amount of time.
Some people believe that before Atlantis, there was an even earlier proto-civilization called Mu, or Lemuria, which in some way incurred the wrath of the Gods and was duly sunk beneath the waves of the Pacific, in the approved fashion of divine retaliation... more rational thinkers have suggested that this was a Victorian invention, added for completion and balance, as the Atlantic already had its own sunken proto-civilization, so why shouldn't the Pacific? A more cynical theory is that survivors of fabled Mu fled to the Atlantic and started a new civilization on a handy centrally located island, which meant that when the Gods noticed they'd missed a few Lemurians, Atlantis was doomed from day one...
The name Lemuria entered the lexicon of the Occult through the works of Madame Blavatsky, who claimed in the 1880s to have been shown an ancient, pre-Atlantean Book of Dzyan by the Mahatmas. Within Blavatsky's complex cosmology, which includes seven "Root Races", Lemuria was occupied by the "Third Root Race", which was about seven foot tall, sexually hermaphroditic, egg-laying, mentally undeveloped and spiritually more pure than the following "Root Races". Before the coming of the Lemurians, the second "Root Race" is said to have dwelled in Hyperborea.
After the subsequent creation of mammals, Mme Blavatsky revealed to her readers, some Lemurians turned to bestiality. The gods, aghast at the behavior of these "mindless" men, sank Lemuria into the ocean and created a "Fourth Root Race"—endowed with intellect—on Atlantis.
According to L. Sprague de Camp, Mme Blavatsky was influenced by other writers on the theme of Lost Continents, notably Ignatius L. Donnelly, a cult leader named Thomas Lake Harris and the French writer Louis Jacolliot.
According to Robert Anton Wilson and Robert Shea, in the Illuminatus! trilogy of tongue-in-cheek conspiracy fiction/fantasy novels, a few survivors of Mu live on in the world as well-concealed immortals with supreme powers of vision and hearing, steering and guiding its fortunes from a secret kingdom in the fastness of the Himalayas. These are The Justified Ancients of Mummu, who have also been immortalised in a song by Tammy Wynette and the K.L.F. (The K.L.F. take their name from Shea and Wilson, and could be called a tribute band to the gleefully anarchic spirit of the trilogy). There may even be a faint and distant echo of the Justified Ancients in a Discworld order of wizardry..
The only other location on Roundworld that took a long time to sink would have to be Venice.