The green liquid in the basin

[pippin_999]

No: HPFGUIDX 165700

> Carol again:
 Voldie himself
> wouldn't need to drink it. He'd know how to get to the (fake) Horcrux
> without doing so. The only point in having the potion there is to
> deter anyone from stealing the Horcrux and force them to drink the
> water so that the Inferi would attack them. And, of course, RAB would
> *want* Voldemort to find his note. It was, after all, addressed to
> him. The only explanation that makes sense to me is for the basin to
> refill itself magically when an object is placed in it.

Pippin:
This makes it even clearer to me that RAB didn't steal the locket
from the basin at all. He stole it beforehand. That eliminates a
whole raft of problems.  RAB doesn't have to be a white wizard 
on a level with Dumbledore in order to penetrate the cave, or a dark
wizard on a level with Voldemort in order to replace the potion. 
He only has to be good enough at transfiguration to make a fake 
locket that would fool Bella, something like the swap Harry did
with his potions books but more sophisticated.

RAB wouldn't have had to count on his transfiguration
being good enough to fool Voldemort himself,  but it just might 
have been, at least for the brief amount of time needed for an 
unsuspecting Voldemort to recover the supposed horcrux from 
Bella and hide it in the cave. Of course if Bella is the one who
did the hiding, so much the better, and Voldemort never saw
the fake at all.

Now,  RAB would have to be exceptional at transfiguration for
this to work, better than Slughorn whose transfiguration was 
detected by Dumbledore almost instantly. But Regulus was one 
of a set with Sirius, and Sirius, very conveniently, was a whiz at
transfiguration, having mastered the immensely difficult and
dangerous animagus transformation at the age of fifteen and
boasting also of knowing everything in his transfiguration 
textbook. 

Really, what would  the point of the fake locket be  if RAB stole
the horcrux from the cave? RAB would have had to have brought
it with him, extracted the real one from the basin and left the
fake, but why? 

To trick Voldemort into drinking the (possibly altered) potion?
Maybe, but as Carol has just pointed out, there's no guarantee that
Voldemort needs to drink the potion at all in order to get the 
locket out.

To conceal the theft a little while longer? But it's not like there
are security guards checking. Only Voldemort is likely to penetrate
the cave. But he will detect traces of others' magic long before
he reaches the island (as Dumbledore should have done if
Regulus had indeed entered the cave before him) and once
his suspicions were roused a fake wouldn't fool him for long.

No, the fake locket only makes sense to me if the substitution was done
earlier, while the locket was still in the possession of Bellatrix. Of
course we do have to account for the locket being disenchanted by the
time Harry found it. But the spell may have worn off over the years, 
or perhaps Dumbledore disenchanted it. Maybe not all the spells he was
muttering were for lifting the protections on Hogwarts, or perhaps
he did indeed survive the fall and discovered the substitution then.

> Carol:
> But DD's argument makes no sense at all. How can Voldemort know that
> someone is there in the cave stealing the Horcrux?

Pippin:
Harry thinks the Inferi are going to drag him into the lake and drown
him. But Inferi don't need to drown people to kill them!
The 'bloody mass'  shown in Snape's classroom clearly wasn't drowned. 

I suspect that once the intruder(s) had been overcome, the Inferi had 
instructions to deliver their catch to Voldemort alive or notify him in some
fashion.

Mike:\
> > Next, DD says "... this potion is supposed to be drunk." Huh? How
> does the potion know that it's being taken out a cupfull at a time
> and being drunk? Conjure a pail and pour it into that! Don't drink the
> stuff! Where's the logic that DD "can only conclude" that drinking is
> the solution?

Pippin:
Because that's what you do with potions! Clearly Voldemort put the
potion there as a challenge, like the protections around the Stone. 
He would have arranged it so that there wasn't any simple way of
retrieving the locket otherwise. 

JKR could have had Dumbledore make more than a cursory effort
to confirm this, but that would be rather dull for us readers, seeing
as how in the end he's bound to drink the potion so the story can
take its course.

Pippin