Most unlikely among the Mysteries of Troll County, perhaps all of Trolldom:
As if the construction of the Great Pyramids of Giza in Egypt, still standing proud after some five millennia, was something of a mystery in and of itself, consider an underwater lodge maintained by water trolls in a remote “divin’ hole” spring somewhere in Troll County’s springs region (and you thought the springs of central Florida were crazy enough, led by Silver and Weeki Wachee in particular).
Built under rather mysterious procedures and conditions, and oft rehabbed by the local water trolls (who, it must be said, have to be pretty tough diver types to keep such in good repair and standing at such depths as 30 feet below surface, considering the water pressure and flow at such a depth), the Underwater Lodge is itself part of Trollkin lore of the highest order, popular among as much young Trolls going through their Wanderjähr exercise as among the trollventurous (including, no doubt, Blitz Lumpkin and Pixlee Trollsom, as have enjoyed the underwater hospitrollity on several occasions).
Yet for all the views into the underwater world of this particular spring (access to which requires knowledge of the local water trolls maintaining same) so afforded, access unto same is something of a trollventure: Trolls so trollsperiencing actually have to dive underwater to reach the lodge, many preferring freediving and some preferring ScubaTroll (and one of the water trolls guiding the way from the beach besides) ... and what’s more, belongings are stashed into special water-tight rucksacks carried along as part of the journey.
But as they say, the reward is the trollsperience of actually looking inside a spring’s depths, and the trollquatic life so inherent (including at least one performance of Troll-flavoured underwater dance, something Trolls as have actually stayed there admit is something not to be missed, as well as a diving session originating from the lodge itself). Much of the troll lore doesn’t exactly elaborate on what inspired the Underwater Lodge to begin with, but for the water trolls of those parts, such is something they can’t help but share the hospitrollity of the depths with.