  We examine a stationary but non-static asymptotically AdS_3 spacetime with
two causally connected conformal boundaries, each of which is a ``null
cylinder'', namely a cylinder with a null direction identified. This spacetime
arises from three different perspectives: (i) as a non-singular, causally
regular orbifold of global AdS_3 by boosts, (ii) as a Penrose-like limit
focusing on the horizon of extremal BTZ black holes, and (iii) as an S^1
fibration over AdS_2. Each of these perspectives sheds an interesting light on
holography. Examination of the conformal boundary of the spacetime shows that
the dual to the space should involve DLCQ limits of the D1-D5 conformal field
theory. The Penrose-like limit approach leads to a similar conclusion, by
isolating a sector of the complete D1-D5 CFT that describes the physics in the
vicinity of the horizon of an extremal black hole. As such this is a
holographic description of the universal horizon dynamics of the extremal black
holes in AdS_3 and also of the four and five dimensional stringy black holes
whose states were counted in string theory. The AdS_2 perspective draws a
connection to a 0+1d quantum mechanical theory. Various dualities lead to a
Matrix model description of the spacetime. Many interesting issues that are
related to both de Sitter physics and attempts to ``see behind a horizon''
using AdS/CFT arise from (a) the presence of two disconnected components to the
boundary, and (b) the analytic structure of bulk physics in the complex
coordinate plane.
