The pool occupies a sink hole in the underlying limestone that has become sealed with glacial boulder clay, preventing the water from seeping into the dissolved rock caverns beneath it and has been known to dry out completely only twice - the first time in 1897 and the second in the 1920's.
(The pool; occupies; a sink hole) Context(The pool occupies,List([0, 17))):(The pool; occupies a sink hole preventing; the water; from seeping into the dissolved rock caverns beneath it) (the underlying limestone; has become; sealed with glacial boulder clay)
The pool occupies,List([0, 17))
The pool The pool the underlying limestone
occupies occupies a sink hole preventing has become
a sink hole
the water from seeping into the dissolved rock caverns beneath it
sealed with glacial boulder clay