Dark, Stygian waters?

Being a Thursday, it ought to be Jerra day - BUT - last two Thursdays he's been down with the (NON-covid), chesty, Lurgi and daren't come through for fear of Smittling me.  Clicky postulated "a working hypothesis", to whit (To Whoo) meet at Melki, that way we could put the world to rights while still "socially distant", hopefully avoiding handing on the Lurgi baton..
It came to pass that we both shot this every visit and, it seems, neither of us have ever Blipped it.  Time one of us did, will we both this time?  
As we approached there was no hint of reflection, a matt-black metal sheet could have been substituted in our absence.  The reflection only appeared as we neared.  We'd neither of us ever seen it free from Algae and for all the depth we could make out it could be any depth twixt 1cm and 1m, or much more, though I seriously doubt it.