I assume you are asking the deepest structure of any sort that exists. Here is the info:

Assorted Substructures:

One mystery undersea structure:

Cuban Underwater City

The Cuban Underwater City is another underwater structure that raises more questions than it answers. What appears to be ruins of a submerged city with corridors and buildings including massive granite complexes, pyramids and circular structures is located off the coast of western Cuba as much as 2,000 feet (650 meters) below the sea level. According to the scientists, it would take about 50,000 years for the alleged city to be submerged to its current depth. As a result, many expressed doubt that the researchers from a Canadian company really discovered a lost ancient city in 2001. Unfortunately, sonar and underwater video robot images are everything that the researchers have so far about the site. Until more is known, it is impossible to say what is down there although the possibility that it is really a lost ancient city cannot be ruled out either.

https://historylists.org/other/list-of-5-most-mysterious-underwater-structures.html

Note: THe deepest descent in the ocean by human

"1960: Jacques Piccard and Don Walsh, with the deep sea vessel Trieste, descend to the bottom of the Challenger Deep in the Mariana Trench (10,740 meters/6.67 miles). They observe fish and other organisms. Fish were not thought to inhabit such deep water."

https://www.thoughtco.com/deep-sea-exploration-4161315#:~:text=1%20The%20deepest%20part%20of%20the%20Earth%27s%20oceans,of%20extra%20space%20above%20it.%20More%20items...%20

I assume you are asking the deepest structure of any sort that exists. Here is an infographic :

_{Source : https://alansfactoryoutlet.com/the-deepest-underground-structures-in-the-world/}

One mystery undersea structure:

Cuban Underwater City

The Cuban Underwater City is another underwater structure that raises more questions than it answers. What appears to be ruins of a submerged city with corridors and buildings including massive granite complexes, pyramids and circular structures is located off the coast of western Cuba as much as 2,000 feet (650 meters) below the sea level. According to the scientists, it would take about 50,000 years for the alleged city to be submerged to its current depth. As a result, many expressed doubt that the researchers from a Canadian company really discovered a lost ancient city in 2001. Unfortunately, sonar and underwater video robot images are everything that the researchers have so far about the site. Until more is known, it is impossible to say what is down there although the possibility that it is really a lost ancient city cannot be ruled out either.

https://historylists.org/other/list-of-5-most-mysterious-underwater-structures.html

Note: THe deepest descent in the ocean by human

"1960: Jacques Piccard and Don Walsh, with the deep sea vessel Trieste, descend to the bottom of the Challenger Deep in the Mariana Trench (10,740 meters/6.67 miles). They observe fish and other organisms. Fish were not thought to inhabit such deep water."

https://www.thoughtco.com/deep-sea-exploration-4161315#:~:text=1%20The%20deepest%20part%20of%20the%20Earth%27s%20oceans,of%20extra%20space%20above%20it.%20More%20items...%20

I assume you are asking the deepest structure of any sort that exists. Here is the info:

Assorted Substructures:

I assume you are asking the deepest structure of any sort that exists. Here is the info:

Assorted Substructures:

One mystery undersea structure:

Cuban Underwater City

The Cuban Underwater City is another underwater structure that raises more questions than it answers. What appears to be ruins of a submerged city with corridors and buildings including massive granite complexes, pyramids and circular structures is located off the coast of western Cuba as much as 2,000 feet (650 meters) below the sea level. According to the scientists, it would take about 50,000 years for the alleged city to be submerged to its current depth. As a result, many expressed doubt that the researchers from a Canadian company really discovered a lost ancient city in 2001. Unfortunately, sonar and underwater video robot images are everything that the researchers have so far about the site. Until more is known, it is impossible to say what is down there although the possibility that it is really a lost ancient city cannot be ruled out either.

https://historylists.org/other/list-of-5-most-mysterious-underwater-structures.html

Note: THe deepest descent in the ocean by human

"1960: Jacques Piccard and Don Walsh, with the deep sea vessel Trieste, descend to the bottom of the Challenger Deep in the Mariana Trench (10,740 meters/6.67 miles). They observe fish and other organisms. Fish were not thought to inhabit such deep water."

https://www.thoughtco.com/deep-sea-exploration-4161315#:~:text=1%20The%20deepest%20part%20of%20the%20Earth%27s%20oceans,of%20extra%20space%20above%20it.%20More%20items...%20