Final answer:
The subject of the sentence is 'dirt'.
Step-by-step explanation:
The subject of the sentence 'Huge mounds of dirt surround the construction site' is dirt.
In this sentence, 'huge' and 'mounds' are adjectives that describe the dirt, while 'site' is the object being surrounded by the dirt. The subject of a sentence is typically the noun that performs the action or is being described. In this case, it is 'dirt' that is being described as 'huge.' So, the subject of the sentence is 'dirt'.