Oh that is such a challenge.

"Oh?"

"It's got layers an' layers o' magic, goblin magic, an' even dragons though I ain't never gone deep enough to see 'em."

A pair of goblins bow them through the silver doors and they're in a vast marble hall. About a hundred more goblins are sitting on high stools behind a long counter, scribbling in large ledgers, weighing coins in brass scales, examining precious stones through eyeglasses. There are too many doors to count leading off the hall, and yet more goblins are showing people in and out of these. Hagrid makes for the counter.

"Morning," he says to a free goblin. "We've come ter take some money outta Ms. Dorea Potter's safe."

"You have her key, sir?"

"Got it here somewhere," says Hagrid, and he starts emptying his pockets onto the counter, scattering a handful of moldy dog biscuits over the goblin's book of numbers. The goblin wrinkles his nose.

Okay that's kind of gross.

She turns over what he said in her mind.

"Got it!" he says at last, holding up a tiny golden key.

The goblin looks at it closely. "That seems to be in order."

"An' I've also got a letter here from Professor Dumbledore," he says importantly, throwing out his chest. "It’s about the You-Know-What in vault seven hundred and thirteen."

The goblin reads the letter carefully. "Very well," he says, handing it back to Hagrid.

"...Did you say you-know-what because you don't want other people to know what, or is it another scary thing like 'you-know-who'?"

"Firs' thing."

The goblin peers down at Dorea suspiciously but doesn't comment. "I will have someone take you down to both vaults. Griphook!"

Griphook is, apparently, yet another goblin. Hagrid follows him to a door, and he holds it open for them. On the other side, there is a narrow stone passageway lit with flaming torches. It slopes steeply downward and there are little railway tracks on the floor. Griphook whistles and a small cart comes hurtling up the tracks toward them. Hagrid climbs in with some difficulty.

Dorea has an easier time of it.

They're off.

At first they just hurtle through a maze of twisting passages. Left, right, right, left, middle fork, right, left, it soon becomes untenable to keep track of the path. The rattling cart seems to know its own way, because Griphook isn't steering.

Cold air rushes past them, and at one point there's something that looks a lot like a burst of fire at the end of a passage but they plunge deeper too fast for much more detail to be made out. They pass an underground lake where huge stalactites and stalagmites grow from the ceiling and floor.

When the cart stops at last beside a small door in the passage wall, Hagrid gets out and has to lean against the wall to stop his knees from trembling. He's looking positively green.

"Are you okay?"

"I'll be fine," he grumbles.

Griphook unlocks the door. A lot of green smoke comes billowing out, and as it clears, Dorea can see the vault's contents: mounds of gold coins, columns of silver, heaps of little bronze Knuts.

"All yours," smiles Hagrid weakly. 

"Whoah."

"Yeh can get as much as yeh like."

"How much am I likely to need? I...don't know what things are worth how much, here."

"I'll get yeh about twice as much as yeh'll need," he suggests, and starts piling some into a bag, eyeballing it more than counting it. "The gold ones are Galleons," he explains. "Seventeen silver Sickles to a Galleon and twenty-nine Knuts to a Sickle, it's easy enough."

"Huh, okay."

He turns to Griphook. "Vault seven hundred and thirteen now, please, and can we go more slowly?"

"One speed only," says the goblin.

They board, and the cart starts again. They go even deeper now and gather speed. The air becomes colder and colder as they hurtle round tight corners. They go rattling over an underground ravine, deep enough the bottom's out of sight.

...Dorea holds on really really tightly.

They soon reach vault seven hundred and thirteen... which has no keyhole.

"Stand back," says Griphook importantly. He strokes the door gently with one of his long fingers and it simply melts away. "If anyone but a Gringotts goblin tried that, they'd be sucked through the door and trapped in there," the goblin says.

Huh. Interesting.

Griphook stands out of the way, and inside of the vault there is...

Nothing.

Or, wait, there's a grubby little package wrapped up in brown paper lying on the floor, there. Hagrid picks it up and tucks it deep inside his coat.

"Come on, back in this infernal cart," says the giant.

...She's not going to ask what that is. Hagrid already said he didn't want anyone to know.

But damn she wants to know.

One wild cart ride later they stand in the sunlight outside Gringotts.

"Might as well get yer uniform," says Hagrid, nodding toward Madam Malkin's Robes for All Occasions. "Listen, Dorea, would yeh mind if I slipped off fer a pick-me-up in the Leaky Cauldron? I hate them Gringotts carts." He does still look a bit green around the gills.

"No, go ahead," she says.

"I won't be long."

Off he goes.

She goes into the shop.