Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. "Ceaselessly" describes how something happens, in this case, it describes the action of a verb.
* Adverbs often end in "-ly". This is a common way to form adverbs in English.
For example:
* The river flowed ceaselessly towards the ocean. (Modifies the verb "flowed")
* The ceaselessly working machine never stopped. (Modifies the adjective "working")