In documents obtained by TMZ, the family of Paul-Henri Nargeolet -- a French explorer who had traveled to see the Titanic wreck dozens of times before the implosion -- filed the lawsuit ... accusing OceanGate of gross negligence.

The family says OceanGate designed and operated Titan outside the norms of the diving community ... and, ultimately they say Paul's life was lost due to the company's gross negligence.

While the family acknowledged Nargeolet was a crewmember, they say many of the vessel's issues were concealed from him ... so, he had no idea how dangerous the dive was.

In the lawsuit, the family points out the crew experienced "mental anguish" ... having tried to abort the dive approximately 90 minutes into the descent ... so, they would've known they were going to die before they actually did.

Nargeolet's family's suing for gross negligence, mental anguish and more ... and, they're seeking damages.

The Titan submersible dove last June ... and quickly lost contact with the surface. After many days of desperate rescue attempts and waiting, it was revealed the sub imploded not long after the dive commenced.

Some experts estimated those on the submersible may have realized they were going to die between 48 and 71 seconds before the implosion.

Along with Nargeloet ... OceanGate CEO and sub pilot Stockton Rush, British billionaire Hamish Harding, Pakistani businessman Shahzada Dawood and his son Suleman all died in the tragedy.