If you have, call the nearest highway or parks department and have them poison it. It’s Giant Hogweed. It’ll grow over 8 feet tall, and the sap in its stalk reacts with sunlight on your skin to cause bad cases of photo-dermatitis. It’s a really bad plant; the skin disfiguration from it lasts years. The clusters of white flowers are each about a foot across. It’s a big plant.
It’s also one more example of an invasive species. This particular plant was imported from Asia as an ornamental, and like many foreign invaders, spread way beyond what was anticipated. It is a public health hazard.
There’s some growing just off the road within a few hundred yards of my house. I called the town to hopefully get rid of it by tomorrow, before any kids get into it.