The green liquid in the basin

[pippin_999]

No: HPFGUIDX 165715

Pippin:
My theory, which I apparently didn't explain clearly enough,  is that 
Regulus learned of the horcrux while Bella still had it. He
transfigured a locket to resemble Slytherin's, stole Slytherin's 
locket from Bella and left the other in its place before it was
hidden in the cave, leaving Bella and Voldemort none the wiser.

Dumbledore discovered the substitution and disenchanted the locket
after he'd scooped it from the basin.

Carol:
 No Transfiguration required; he just wants to fool the basin,
> not Bellatrix, who would be extremely stupid if she mistook a plain
> locket for the Slytherin heirloom that LV had entrusted to her keeping.

Pippin:
But how would Regulus know what it would take to fool the basin?
Kreacher wouldn't know that. Besides, no magic is need to explain
Kreacher's mental condition: IMO, he is, or was,   a normal
House Elf who  was imprisoned in an abandoned house for years,
ever since Mrs. Black's death, with no one but a mad old portrait 
to talk to. Who wouldn't be a little barmy under those circumstances?

To take away from that undermines the irony that Grimmauld
Place was no healthier for him than it was for Sirius. 

Carol:
That problem is resolved by an enchantment making the basin 
fill or refill itself when an object is placed in it.

Pippin:
Then the basin would have refilled when Harry dropped the goblet
in it:

"No!" shouted Harry, who had stood to refill the goblet again;
instead he dropped the cup into the basin, flung himself down 
beside Dumbledore, and heaved him over onto his back. --HBP
ch 26.

> 
> Pippin:
> >
> > 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.
> 
> Carol:
> If that's the case, they'd have notified Voldie that the original
> locket was stolen (unless your substitute locket theory is true, and I
> don't think so for reasons already stated). Nor do the Inferi do
> anything besides slide back into the lake hiding from the fire after
> Harry and DD steal the fake Horcrux. No alarm bells. No Inferi casting
> the DE equivalent of Patronus Charms to notify LV. No runaway Inferi
> reported heading toward Little Hangleton. And how would they get out
> of the cave and down the cliffside, anyway?

Pippin:

I don't think the Inferi can do much thinking for themselves.
 If Voldemort doesn't think any intruder can escape his trap, 
he might have neglected to instruct the Inferi to notify him
if someone grabs the locket and escapes them. They probably 
only have instructions to notify him if they've captured a thief.
A little of the intruder's  blood will enable them to leave the cave.

Pippin