Blue grass was a type of grass that grew in 2020 Wales. It turned blue because of minerals being unearthed by Silurians. The grass was a warning from the Silurians for humans not to disturb, but the humans weren't aware of this, so the Silurians started drilling upwards to try and stop them. This got the attention of the Eleventh Doctor, who realised something was wrong. (TV: The Hungry Earth)
