The protection measures at Heimaey are undoubtedly the most extensive that have ever been used in a volcanic eruption.
(The protection measures at Heimaey; are undoubtedly; the most extensive that have ever been used in a volcanic eruption) (the most extensive; have been used; L:in a volcanic eruption; T:ever)
The protection measures at Heimaey the most extensive
are undoubtedly have been used
the most extensive that have ever been used in a volcanic eruption
L:in a volcanic eruption T:ever