TITLE: Intuition for Zagier's theorem for $\zeta_K(2)$
QUESTION [29 upvotes]: In 1986, Don Zagier generalized Euler's theorem ($\zeta_\mathbb{Q}(2)=\pi ^2 /6$) to an arbitrary number field $K$:
$$\zeta_K(2)=\frac{\pi^{2r+2s}}{\sqrt{|D|}}\times \sum_v c_v A(x_{v,1})...A(x_{v,s})$$
where
$$A(x)=\int^x_0 \frac{1}{1+t^2}\log \frac{4}{1+t^2}dt$$
In this sense, the result is conjectured to hold for $2<s\in \mathbb{N}$, with the $A(x)$ replaced by more complicated functions $A_m(x)$
This might seem rather unenlightening, but we can also state Zagier's result like this:

$$\zeta_K(2)=\text{the volume of a hyperbolic manifold}$$

This amazing fact doesn't seem to have a direct analogue for $\zeta_K(2m)$ with $m \neq 1$

What I'd like to know is if there is any big picture explanation for the appearance of hyperbolic manifolds in this context.

Zagier's calculation is quite geometrical, but as far as I understand gives no clear explanation of "what the manifold is doing here".
Don Zagier, Hyperbolic manifolds and special values of Dedekind zeta-functions (1986)

REPLY [12 votes]: There is a K-theory reformulation of Zagier's theorem which explains both the appearance of the integrals $A(x)$ as well as the appearance of hyperbolic geometry. I will try to give a sketch what is behind the relation of the zeta-value with volumes of hyperbolic simplices. 
First, due to the work of Borel, there is a relation between zeta-values and regulators for K-theory. For a number field $K$, the Borel regulator embeds $K_{2n-1}(\mathcal{O}_K)$  as a lattice in some $\mathbb{R}$-vector space (later reinterpreted by Beilinson as Deligne-cohomology group $H^1_{\mathcal{D}}(\operatorname{Spec} K\otimes\mathbb{C},\mathbb{R}(n))$). The covolume of the lattice is, up to multiplication by a power of $\pi$ and an algebraic number, identified with the zeta-value $\zeta_K(-n+1)$. So, Borel's theorem states: The zeta-value is the covolume of the regulator embedding $K_3(\mathcal{O}_K)\to\mathbb{R}^{r_2}$.
In a second step, there is a relation between K-theoretic regulators and dilogarithms (following the work of Bloch, Suslin, Dupont-Sah...). Recall that the pre-Bloch group $\mathcal{P}(K)$ is given by generators $x\in K^\times\setminus\{1\}$ subject to the five-term relation $$[x]-[y]+[y/x]-[(1-x^{-1})/(1-y^{-1})]+[(1-x)/(1-y)]$$ for $x, y\in K^\times\setminus\{1\}$. The Bloch group $\mathcal{B}(K)$ is the kernel of the map
$$
\mathcal{P}(K)\to\bigwedge^2 K^\times:[x]\mapsto x\wedge(1-x).
$$
The five-term relation is a version of the functional equation for the dilogarithm. Hence, by definition, the single-valued real-analytic version of the dilogarithm yields a function mapping the Bloch group into $\mathbb{R}^{r_2}$ by taking a class $[x]$ to the values of the dilogarithm under the various complex embeddings $K\to\mathbb{C}$. The Bloch group can (up to 2-torsion) be identified with the indecomposable quotient of $K_3(\mathcal{O}_K)$. The important thing is that under this identification, the regulator on $K_3$ is mapped to the dilogarithm function on the Bloch group. (There are some factors, algebraic numbers and powers of $\pi$, but this does not cause problems for the identification.)
It is mentioned in Zagier's paper that $A(x)$ is essentially the dilogarithm. The combination of the above two steps yields the reformulation of Zagier's theorem: Expressing the regulator embedding by dilogarithms, the zeta-value is a determinant of dilogarithms of elements of $K$.
In a third step, the relation to hyperbolic geometry can now be obtained from the Bloch group. Via the cross-ratio, the pre-Bloch group is isomorphic to the third homology of the complex of points on $\mathbb{P}^1(K)$ (with the omit-one-point-differential) modulo the diagonal action of $GL_2(K)$. This reinterprets elements of the Bloch group as ordered $4$-tuples of points on $\mathbb{P}^1(K)$. Now four points on $\mathbb{CP}^1$ span a simplex in $\mathbb{H}^3\cup\partial\mathbb{H}^3$. One can talk about hyperbolic $3$-polytopes up to scissors congruence. It is possible to prove that the group of scissors congruence classes in $\mathbb{H}^3$ is isomorphic to the group of scissors congruence classes of polytopes in $\mathbb{H}^3\cup \partial\mathbb{H}^3$ whose vertices lie on $\partial\mathbb{H}^3$ (this is done in the book "Scissors congruences, group homology and characteristic classes" by J.-L. Dupont). 
We can now explain the link between the Bloch group and hyperbolic geometry by: $\partial\mathbb{H}^3\cong \mathbb{CP}^1$. More precisely, the Bloch group is a group of scissors congruence classes of hyperbolic $3$-polytopes with vertices on $\partial\mathbb{H}^3$.
Combining the above three steps means that $\zeta_K(2)$ can be expressed as a combination of volumes of hyperbolic simplices. This statement actually has a conjectural generalization for all $\zeta_K(n)$. Zagier formulated the corresponding conjecture in "Polylogarithms, Dedekind zeta functions and the algebraic K-theory of fields". There are analogues of the Bloch group which conjecturally can be identified with higher K-groups $K_{2n-1}(\mathcal{O}_K)_{\mathbb{Q}}$, such that the regulator on the K-group can be expressed in terms of $n$-logarithms. If true, this conjecture would allow to express $\zeta_K(n)$ as a determinant of $n$-logarithms, or equivalently as a linear combination of volumes of hyperbolic $(2n-1)$-simplices with vertices on the boundary. The trilogarithm case of Zagier's conjecture was proved by Goncharov, see

A. Goncharov. Volumes of hyperbolic manifolds and mixed Tate motives. J. Amer. Math. Soc. 12 (1999), 569-618. 

As a final note, the relation between polylogarithms and volumes of hyperbolic simplices goes back to Lobachevskiy, Schläfli,...