Supplemental Index is one of the two index categories (main index being the first one) where Google places certain webpages due to various flaws encountered in the indexing process.

Although the webpage isn't in the main index it continues to be indexed, as a matter of fact with even FEWER restrictions as opposed to the main index.

The bad news is that this supplemental index is the last stop before the page is one hundred percent dropped by Google.

One important distinction must be made here and this is: Don't confound the SI with Google's most feared Sandbox Effect. The Sandbox effect tends to appear with webmasters who conduct aggressive SEO practices such as intensive link building campaigns, usage as anchor texts of expensive keywords (as judged by Adwords stats), getting involved in doggy link farms and link exchanges in general, etc.

Before I present the major reasons why a webpage slips in the supplemental index, please take note that this is a auto generated algorithm and no human inspector supervises who gets or should get in and vice versa.

What causes a webpage to fall into Google's Supplemental Index:

1. inconsistent internal linking - the most common way a webpage ends up in the SI;

Periodically Google conducts a deep crawl for your site. When it does this, it also compares the new findings with the information gathered in previous inspections or crawls.

If Google finds out that some of the previously indexed webpages don't appear in the new data report for your site, it automatically presumes that you've abandoned them.. These webpages are qualified to be moved in supplemental index.

What course of action to take in this situation? Quite simple. Initiate a link building campaign and let Google know that your webpages are still of interest for it's users. Web 2.0 platforms are a good starting point for your link building campaign.

2. the link between the home/category page and the relegated webpage is MISSING;

The answer to these scenario is fairly predictable; just build a link on the category or home page pointing to the missing segment of your website and give Google a week to re-index your site.

3. webpages simply lack updates after being live and indexed for more then twelve months - this cause generally appears with thin websites;

What you should do: rewrite the headings and the title tag. If the content is outdated, consider rewriting the entire body text. In a nutshell, focus on creating new, optimized content.

Other shortcuts to reclaim your de-indexed webpages:

Constantly add your website to good submission directories. If you can afford submit to Yahoo Directory and try your luck with DMOZ;

Write and submit press releases. The paid PR directories like PRWeb, PRLeap work best;

When building back links keep in mind the "quality" factor over the volume. Some article directories satisfy both needs. Needless to say the EzineArticles and Article Bases are the AD to work with;

Optimize your on-page factors.

The truth is that Google's supplemental index could negatively impact your site's overall authority and rankings if isolated webpages fall in it one after the other. So be sure to both rescue and prevent Supplemental Indexation from happening with the methods presented above.