diff --git a/.gitignore b/.gitignore
index d6b4306ef..9376efc76 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,3 +7,4 @@ flamegraph.svg
 perf.data*
 *.tar.gz
 /bin
+genesis.ssz
diff --git a/common/eth2_network_config/build.rs b/common/eth2_network_config/build.rs
index f6a07171f..2879925dd 100644
--- a/common/eth2_network_config/build.rs
+++ b/common/eth2_network_config/build.rs
@@ -1,5 +1,4 @@
-//! Downloads a network configuration from Github.
-
+//! Extracts zipped genesis states on first run.
 use eth2_config::{
     altona, mainnet, medalla, pyrmont, spadina, toledo, Eth2NetArchiveAndDirectory,
     GENESIS_FILE_NAME,
@@ -31,7 +30,16 @@ fn main() {
 
 /// Uncompress the network configs archive into a network configs folder.
 fn uncompress_state(network: &Eth2NetArchiveAndDirectory<'static>) -> Result<(), String> {
+    let genesis_ssz_path = network.dir().join(GENESIS_FILE_NAME);
+
+    // Take care to not overwrite the genesis.ssz if it already exists, as that causes
+    // spurious rebuilds.
+    if genesis_ssz_path.exists() {
+        return Ok(());
+    }
+
     if network.genesis_is_known {
+        // Extract genesis state from genesis.ssz.zip
         let archive_path = network.genesis_state_archive();
         let archive_file = File::open(&archive_path)
             .map_err(|e| format!("Failed to open archive file {:?}: {:?}", archive_path, e))?;
@@ -45,18 +53,14 @@ fn uncompress_state(network: &Eth2NetArchiveAndDirectory<'static>) -> Result<(),
                 GENESIS_FILE_NAME, e
             )
         })?;
-        let path = network.dir().join(GENESIS_FILE_NAME);
-        let mut outfile = File::create(&path)
-            .map_err(|e| format!("Error while creating file {:?}: {}", path, e))?;
+        let mut outfile = File::create(&genesis_ssz_path)
+            .map_err(|e| format!("Error while creating file {:?}: {}", genesis_ssz_path, e))?;
         io::copy(&mut file, &mut outfile)
-            .map_err(|e| format!("Error writing file {:?}: {}", path, e))?;
+            .map_err(|e| format!("Error writing file {:?}: {}", genesis_ssz_path, e))?;
     } else {
         // Create empty genesis.ssz if genesis is unknown
-        let genesis_file = network.dir().join(GENESIS_FILE_NAME);
-        if !genesis_file.exists() {
-            File::create(genesis_file)
-                .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
-        }
+        File::create(genesis_ssz_path)
+            .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
     }
 
     Ok(())
diff --git a/common/eth2_network_config/built_in_network_configs/altona/genesis.ssz b/common/eth2_network_config/built_in_network_configs/altona/genesis.ssz
deleted file mode 100644
index 7a91759df..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/altona/genesis.ssz and /dev/null differ
diff --git a/common/eth2_network_config/built_in_network_configs/mainnet/genesis.ssz b/common/eth2_network_config/built_in_network_configs/mainnet/genesis.ssz
deleted file mode 100644
index f68d0e967..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/mainnet/genesis.ssz and /dev/null differ
diff --git a/common/eth2_network_config/built_in_network_configs/medalla/genesis.ssz b/common/eth2_network_config/built_in_network_configs/medalla/genesis.ssz
deleted file mode 100644
index 6691c8992..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/medalla/genesis.ssz and /dev/null differ
diff --git a/common/eth2_network_config/built_in_network_configs/pyrmont/genesis.ssz b/common/eth2_network_config/built_in_network_configs/pyrmont/genesis.ssz
deleted file mode 100644
index c566d7e04..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/pyrmont/genesis.ssz and /dev/null differ
diff --git a/common/eth2_network_config/built_in_network_configs/spadina/genesis.ssz b/common/eth2_network_config/built_in_network_configs/spadina/genesis.ssz
deleted file mode 100644
index ccc90a001..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/spadina/genesis.ssz and /dev/null differ
diff --git a/common/eth2_network_config/built_in_network_configs/toledo/genesis.ssz b/common/eth2_network_config/built_in_network_configs/toledo/genesis.ssz
deleted file mode 100644
index a5b588a9f..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/toledo/genesis.ssz and /dev/null differ
diff --git a/common/eth2_network_config/built_in_network_configs/zinken/genesis.ssz b/common/eth2_network_config/built_in_network_configs/zinken/genesis.ssz
deleted file mode 100644
index f645a7f24..000000000
Binary files a/common/eth2_network_config/built_in_network_configs/zinken/genesis.ssz and /dev/null differ