Huge cave system under Manitoulin Island

While watching the Great Lakes surface temperatures (site currently unavailable due to government shutdown) this past summer, I noticed the surface water temperatures of Lake Huron just south of the western end of Manitoulin Island had several jets of surface water colder than the surface waters in any of the lakes except Lake Superior.  Where did that cold water come from?  It must flow up from lower, hence colder levels of Lake Huron or originate in Lake Superior - or both.  Flowing up from more southerly, lower levels doesn't make sense. If that were happening at the south shore of Manitoulin Island, why not at the the northern shore of all of the Great Lakes?

What is special about Manitoulin Island that it would have these jets of cold water pouring into Lake Huron?

As part of the Niagara Escarpment, Manitoulin Island is limestone.  Limestone easily erodes in water.  Often cave systems are formed.  The Mammoth Cave system in Kentucky was formed by water flowing from the north through a porous limestone escarpment.

I propose the same thing is happening now at the north end of the Niagara Escarpment, draining the deeper waters of Lake Superior and Georgian Bay into Lake Huron.  The caverns must be vast, and the flow rate torrential enough that the deep cold water displaces even the warm surface water where the jets empty into Lake Huron.