Definition of "unsuspected"

At that time it was thought that the critical section, in which an irruption of water from the river was to be feared, lay under the Shoots, but this proved not to be the case. Indeed, relatively little trouble was experienced from the Severn. The very serious flooding, which twice submerged part of the works, came from an underground river, (now known as the Great Spring), the presence of which was unsuspected.

1948 September and October, “Weekend Works in the Severn Tunnel”, in Railway Magazine, page 299