In Damp Metro Tunnels, Prehistoric Plants Thrive
wamu.org
wamu.org
While the fact that this happens is interesting from a botanical perspective, it's pretty indicative of a larger problem with the infrastructure that allows it to happen, IMO.
These stations also had a tendency to flood during particularly heavy rains, so it's no surprise that questionable drainage is the culprit.
It's interesting to note that these stations get no natural light where the ferns are growing. It's all manmade light.
"As a result, he said, they “discharge approximately 2-million gallons of water a day.” In other words: about three Olympic sized swimming pools worth."