diff --git a/InstallDetails.md b/InstallDetails.md
index fa4d45c..15d37d5 100644
--- a/InstallDetails.md
+++ b/InstallDetails.md
@@ -40,6 +40,39 @@ symlink). So until you delete the source tree, you can
 symlink `~/bin/tahoe` to your source tree's `bin/tahoe` and then use
 it as you would any other system executable.
 
+## the "Desert Island" Build
+
+Tahoe will download and install most of the libraries it requires when you
+run "`make`". You might want to pre-download these libraries: perhaps you
+are about to get on an airplane, or you anticipate having poor network
+connectivity, or you just don't like the idea of a so-called compile step
+using the network (the download step uses !PyPI to figure out where to
+download these libraries from, so you might be concerned that it or one of
+the project web pages it references has been modified to point at something
+malicious).
+
+This disconnected-build operation is supported in two ways. When building
+from a darcs checkout, you can download the latest "tahoe-deps" bundle from
+<http://allmydata.org/source/tahoe/deps/tahoe-deps.tar.gz> . Unpack this in
+your source tree, and the build process will grab any necessary libraries
+from its `tahoe-deps/` directory instead of downloading them from the
+internet. You can also unpack tahoe-deps into the parent directory to share
+it between multiple trees.
+
+Alternatively, if you are building from a tarball (either a release tarball
+from <http://allmydata.org/source/tahoe/releases/> , or a continually-generated
+current-trunk tarball from <http://allmydata.org/source/tahoe/tarballs/> ), you
+can simply download the -SUMO version of the tarball instead of the regular
+one. The "SUMO" tarball includes the current tahoe-deps bundle pre-unpacked
+in the source tree.
+
+The tahoe-deps bundle is updated every once in a while, as new versions of
+the dependent libraries are released, or as Tahoe becomes dependent upon new
+things. The <http://allmydata.org/source/tahoe/deps/> directory contains
+historical ones, but there should never be a reason to use anything but the
+latest. The tahoe-deps bundle contains a README that has a version number.
+
+
 ## Installing Outside The Source Tree
 
 If you want to use Tahoe without keeping the source tree around, you will