Blue grass
From Tardis Wiki, the free Doctor Who reference
Blue grass was a form of grass that grew in 2020 Wales. It turned blue because of minerals being unearthed by Silurians. The Blue 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 the drilling. This got the attention of the Eleventh Doctor, who realised something was wrong. (DW: The Hungry Earth)
