The answer to the question above would be that the sentence that has an elliptical adverb clause with an understood word or word is the following one: As we travel higher than does the sea level, the boiling point of water drops.
The elliptical clause is a type of dependent or subordinate clause that is missing a word or words. Usually, the missing part is a verb or verb phrase. This is the case in the sentence above: The verb in the comparative clause is missing and the auxiliary has been added to refer to the action described by the verb (travel). That is, the complete sentence would be as follows: As we travel higher than the sea level travels". The fact is that this has also been done in order to avoid repetition.
