Pennywise and Maturin

Mia states that "the world" rests on Maturin's shell:

There are six Beams, as you did say, but there are twelve Guardians, one for each end of each Beam. This—for we’re still on it—is the Beam of Shardik. Were you to go beyond the Tower, it would become the Beam of Maturin, the great turtle upon whose shell the world rests. -Song of Susannah

It's later confirmed that "world", in this case, is referring to all of creation, as Gan made it. After Gan made creation and moved on, Maturin caught it on his back to prevent it from falling into the abyss:

“Gan bore the world and moved on,” Roland replied. “Is that what you mean to say?”
“Aye, and the world would have fallen into the abyss if not for the great turtle. Instead of falling, it landed on his back.” - Song of Susannah

Stephen King's in-universe "writers notes" contain this golden nugget of info:

November 18th, 1984
I had a dream last night that I think breaks the creative logjam on It. Suppose there’s a kind of Beam holding the Earth (or even multiple Earths) in place? And that the Beam’s generator rests on the shell of a turtle? I could make that part of the book’s climax. I know it sounds crazy, but I’m sure I read somewhere that in Hindu mythology there’s a great turtle that bears us all on his shell, and that he serves Gan, the creative overforce. Also, I remember an anecdote where some lady sez to some famous scientist, “This evolution stuff is ridiculous. Everyone knows that a turtle holds up the universe.” To which the scientist (wish I could remember his name, but I can’t) replies, “That may be, madam, but what holds up the turtle?” Scornful laugh from the lady, who says, “Oh, you can’t fool me! It’s turtles all the way down.”
Ha! Take that, ye rational men of science!
Anyway, I keep a blank book by my bed, and have gotten so I write down a lot of dreams and dream elements w/o even fully waking up. This morning I’d written Remember the Turtle! And this: See the TURTLE of enormous girth! On his shell he holds the earth. His thought is slow but always kind; he holds us all within his mind. Not great poetry, I grant you, but not bad for a guy who was three-quarters asleep when he wrote it!
Tabby has been on my case about drinking too much again. I suppose she’s right, but … -Song of Susannah (Coda)

This not only strengthens the idea that Maturin is holding more than just the mainstream SK universe on his back, but also implies that the entirety of the Beams' power source also rests there.

The Beams' power source is none other than the Tower itself:

“There is a Tower, lady and gentlemen, as you must know. At one time six beams crisscrossed there, both taking power from it—it’s some kind of unimaginable power-source—and lending support, the way guy-wires support a radio tower. Four of these Beams are now gone, the fourth very recently. The only two remaining are the Beam of the Bear, Way of the Turtle— Shardik’s Beam—and the Beam of the Elephant, Way of the Wolf—some call that one Gan’s Beam. -The Dark Tower

The Turtle is older and more powerful than It:

But there was something else here. Bill sensed it, felt it, in a crazy way smelled it: some large presence ahead in the dark. A Shape. He felt not fear but a sense of overmastering awe; here was a power which dwarfed Its power, and Bill had only time to think incoherently: Please, please, whatever You are, remember that I am very small —
He rushed toward it and saw it was a great Turtle, its shell plated with many blazing colors. Its ancient reptilian head slowly poked out of its shell, and Bill thought he felt a vague contemptuous surprise from the thing that had cast him out here. The eyes of the Turtle were kind. Bill thought it must be the oldest thing anyone could imagine, older by far than It, which had claimed to be eternal. -IT

Pennywise is stated to come from beyond time and space:

"Then Beverly was shrieking, clinging to Bill, as It raced down the gossamer curtain of Its webbing, a nightmare Spider from beyond time and space, a Spider from beyond the fevered imaginings of whatever inmates may live in the deepest depths of hell.
No, Bill thought coldly, not a Spider either, not really, but this shape isn't one It picked out of our minds; it's just the closest our minds can come to

(the deadlights), whatever It really is." -IT

It views "this universe" as being a "puny egg":

When It had burst up into the house on Neibolt Street, meaning to kill them all, vaguely uneasy that It had not been able to do so already (and surely that unease had been the first new thing), something had happened which was totally unexpected, utterly unthought of, and there had been pain, pain, great roaring pain all through the shape it had taken, and for one moment there had also been fear, because the only thing It had in common with the stupid old Turtle and the cosmology of the macroverse outside the puny egg of this universe was just this: all living things must abide by the laws of the shape they inhabit. For the first time It realized that perhaps Its ability to change Its shapes might work against It as well as for It. There had never been pain before, there had never been fear before, and for a moment It had thought It might die — oh Its head had been filled with a great white silver pain, and it had roared and mewled and bellowed and somehow the children had escaped. -IT

Quote of where It exists:

Suddenly he thought he understood: It meant to thrust him through some wall at the end of the universe and into some other place

(what that old Turtle called the macroverse)

where It really lived; where It existed as a titanic, glowing core which might be no more than the smallest mote in that Other's mind; he would see It naked, a thing of unshaped destroying light, and there he would either be mercifully annihilated or live forever, insane and yet conscious inside Its homicidal endless formless hungry being. -IT