2812500 ft high
A naturally formed rock resembling a pyramid, or some natural ground formation? why not? A large designed engineered and constructed pyramid? No.
Defining the height of a pyramid is by taking a straight line from its base at ground level to its highest point.
under ground chambers
The simile "the coffin sank into the floor as if the ground turned into water" can be found in Chapter 31 of "The Red Pyramid" by Rick Riordan.
The pyramid itself is not underground but one of its three burial chambers is underground and carved into the bedrock.
The pyramid of Khafre is the second pyramid to be built. It is slightly shorter at 430ft than the great pyramid of his father Khufu. In pictures it always looks taller because it was built on higher ground. It is the pyramid with some of its casing stones left at the top.
An Obtuse Angle (Apex)
A square-based pyramid has, let's see, a top vertex, and 4 vertexes on its base (the part in the ground) for a total of 5 vertexes.
The height of the Great Pyramid = 139 metres The slope of a ramp with a mechanical advantage of 4 = 4x139 = 556m
The Step Pyramid of Djoser is located in Saqqara, Egypt. It is part of the Saqqara necropolis, a large burial ground that served as a royal cemetery for the ancient city of Memphis.
In chapter two of the red pyramid Carter and Sadie watch their father gert trapped inside a coffin and sucked into the ground!
because it has sunk to the ground in all these years