Underwear.

But there is a strange reference at Heathrow to the London Underground and in a child's writing, the words "under where?" There is whole exhibit of posters there and many are strange and with references to children.

See Titus Frost, Brittany/Nicole Pettibone, Reality Calls: What's Going On Underground?

It includes Heathrow underground photos with references to children and not just The Underground, but "under ground."

Someone in the UK might want to get copies of all those big posters.

And people in the US might think about checking out what's going on with the subways and water systems - both underground and both being tampered with.