diff --git a/docs/spec/spec.adoc b/docs/spec/spec.adoc
index d3e5f49..2781066 100644
--- a/docs/spec/spec.adoc
+++ b/docs/spec/spec.adoc
@@ -34,4 +34,8 @@ include::structs/header.adoc[]
 
 <<<
 
-include::structs/blocks.adoc[]
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+include::structs/blocks.adoc[]
+
+<<<
+
+include::structs/index2.adoc[]
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diff --git a/docs/spec/structs/index2.adoc b/docs/spec/structs/index2.adoc
new file mode 100644
index 0000000..6d4b3e8
--- /dev/null
+++ b/docs/spec/structs/index2.adoc
@@ -0,0 +1,85 @@
+=== The index block version 2 (`IDX2`)
+
+The index block stores references to all blocks present in the file.
+It is composed of a header, followed by a sequence of entries, the count of which is defined within the header.
+
+Multiple index blocks may exist within a file to represent previous states or historical versions; however, only the final index block must be referenced by the main file header.
+
+==== Structure Definition
+
+[source,c]
+#define INDEX2_MAGIC 0x32584449
+/**Header for the index, followed by entries */
+typedef struct IndexHeader2
+{
+    /**Identifier, <see cref="BlockType.Index" /> */
+    uint32_t identifier;
+    /**How many entries follow this header */
+    uint64_t entries;
+    /**CRC64-ECMA of the index */
+    uint64_t crc64;
+} IndexHeader;
+
+==== Field Descriptions
+
+[cols="2,2,2,6",options="header"]
+|===
+|Type
+|Size
+|Name
+|Description
+
+|uint32_t
+|4 bytes
+|identifier
+|The index block identifier, always `IDX2`
+
+|uint64_t
+|8 bytes
+|entries
+|The number of entries following this header
+
+|uint64_t
+|8 bytes
+|crc64
+|CRC64-ECMA checksum of the entries following this header
+|===
+
+==== Index entries
+
+[source,c]
+/**Index entry */
+typedef struct IndexEntry
+{
+    /**Type of item pointed by this entry */
+    uint32_t blockType;
+    /**Type of data contained by the block pointed by this entry */
+    uint32_t dataType;
+    /**Offset in file where item is stored */
+    uint64_t offset;
+} IndexEntry;
+
+==== Field Descriptions
+
+[cols="2,2,2,6",options="header"]
+|===
+|Type
+|Size
+|Name
+|Description
+
+|uint32_t
+|4 bytes
+|blockType
+|The type of block this entry points to.
+
+|uint32_t
+|4 bytes
+|dataType
+|The type of data the block pointed by this entry contains.
+
+|uint64_t
+|8 bytes
+|offset
+|The offset in bytes from the start of the file where the block pointed by this entry starts.
+|===
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