Amazon Underground is an shopping app that lets you download more than worth $20,000 games and applications for smartphones and tablets absolutely free. These are actually paid apps which Amazon offers absolutely free as an offer time to time, just as they did today for Christmas celebration. Not bad at all.
Now, there are two ways of download Underground app. Remember it is not available on Play Store. Either you can head over to amazon.com/underground and enter your email address to get a download url via web, or directly visit that link from your smartphone or tablet. You will get a url clicking on which the download will start. It will bring you APK file which you will need to install manually.
Or, you can download the Underground APK directly from this page. It takes just one click to get the file on your smartphone. Install it and start downloading the paid apps for free. After installation, if you are not able to see the free underground apps, go to the left menu and change the country to US. You will see the list of free apps and games available for download.