diff --git a/guix/build/git.scm b/guix/build/git.scm
index c1af545a7..223e79227 100644
--- a/guix/build/git.scm
+++ b/guix/build/git.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -17,7 +17,14 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix build git)
+  #:use-module (web uri)
+  #:use-module (web client)
+  #:use-module (web response)
+  #:use-module (guix serialization)
   #:use-module (guix build utils)
+  #:use-module (srfi srfi-11)
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 match)
   #:export (git-fetch))
 
 ;;; Commentary:
@@ -27,6 +34,37 @@
 ;;;
 ;;; Code:
 
+(define (urls-for-item item)
+  "Return the fallback nar URL for ITEM--e.g., \"cabbag3…-foo-1.2-checkout\"."
+  ;; TODO: Use the /gzip URLs, make it configurable, and use TLS.
+  (list (string-append "http://mirror.hydra.gnu.org/guix/nar/" item)
+        (string-append "http://berlin.guixsd.org/nar/" item)))
+
+(define (download-nar item directory)
+  "Download Git checkout ITEM to DIRECTORY as a nar."
+  (setvbuf (current-output-port) _IONBF)
+  (setvbuf (current-error-port) _IONBF)
+
+  (let loop ((urls (urls-for-item item)))
+    (match urls
+      ((url rest ...)
+       (format #t "Trying content-addressed mirror at ~a...~%"
+               (uri-host (string->uri url)))
+       (let-values (((response port)
+                     (http-get url #:streaming? #t)))
+         (if (= 200 (response-code response))
+             (let ((size (response-content-length response)))
+               (if size
+                   (format #t "Downloading from ~a (~,2h MiB)...~%"
+                           url (/ size (expt 2 20.)))
+                   (format #t "Downloading from ~a...~%" url))
+               (restore-file port directory)
+               (close-port port)
+               #t)
+             (loop rest))))
+      (()
+       #f))))
+
 (define* (git-fetch url commit directory
                     #:key (git-command "git") recursive?)
   "Fetch COMMIT from URL into DIRECTORY.  COMMIT must be a valid Git commit
@@ -39,26 +77,27 @@ recursively.  Return #t on success, #f otherwise."
 
   ;; We cannot use "git clone --recursive" since the following "git checkout"
   ;; effectively removes sub-module checkouts as of Git 2.6.3.
-  (and (zero? (system* git-command "clone" url directory))
-       (with-directory-excursion directory
-         (system* git-command "tag" "-l")
-         (and (zero? (system* git-command "checkout" commit))
-              (begin
-                (when recursive?
-                  ;; Now is the time to fetch sub-modules.
-                  (unless (zero? (system* git-command "submodule" "update"
-                                          "--init" "--recursive"))
-                    (error "failed to fetch sub-modules" url))
+  (if (zero? (system* git-command "clone" url directory))
+      (with-directory-excursion directory
+        (system* git-command "tag" "-l")
+        (and (zero? (system* git-command "checkout" commit))
+             (begin
+               (when recursive?
+                 ;; Now is the time to fetch sub-modules.
+                 (unless (zero? (system* git-command "submodule" "update"
+                                         "--init" "--recursive"))
+                   (error "failed to fetch sub-modules" url))
 
-                  ;; In sub-modules, '.git' is a flat file, not a directory,
-                  ;; so we can use 'find-files' here.
-                  (for-each delete-file-recursively
-                            (find-files directory "^\\.git$")))
+                 ;; In sub-modules, '.git' is a flat file, not a directory,
+                 ;; so we can use 'find-files' here.
+                 (for-each delete-file-recursively
+                           (find-files directory "^\\.git$")))
 
-                ;; The contents of '.git' vary as a function of the current
-                ;; status of the Git repo.  Since we want a fixed output, this
-                ;; directory needs to be taken out.
-                (delete-file-recursively ".git")
-                #t)))))
+               ;; The contents of '.git' vary as a function of the current
+               ;; status of the Git repo.  Since we want a fixed output, this
+               ;; directory needs to be taken out.
+               (delete-file-recursively ".git")
+               #t)))
+      (download-nar (basename directory) directory)))
 
 ;;; git.scm ends here
diff --git a/guix/git-download.scm b/guix/git-download.scm
index 7397cbe7f..ffae8fcc3 100644
--- a/guix/git-download.scm
+++ b/guix/git-download.scm
@@ -25,6 +25,7 @@
   #:use-module (guix monads)
   #:use-module (guix records)
   #:use-module (guix packages)
+  #:use-module (guix modules)
   #:autoload   (guix build-system gnu) (standard-packages)
   #:use-module (ice-9 match)
   #:use-module (ice-9 popen)
@@ -78,8 +79,8 @@ HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
         '()))
 
   (define build
-    (with-imported-modules '((guix build git)
-                             (guix build utils))
+    (with-imported-modules (source-module-closure
+                            '((guix build git)))
       #~(begin
           (use-modules (guix build git)
                        (guix build utils)