James Cameron is calling the destruction of the sea sub Nereus a "tragic loss for deep science;" days after the robotic research vessel imploded some 6.2 miles under the sea while exploring the world's second-deepest trench.

"I feel like I've lost a friend," Cameron wrote on the Facebook page of the Woods Hole Oceanographic Institution (WHOI), which was operating the sub. "Nereus was an amazing, groundbreaking robot and the only currently active vehicle in the world that could reach the extreme depths of the ocean trenches."

Cameron, 59, goes on to sympathize with the owners of Nereus, noting his own attachment experiences with the robotic vehicles he used to explore the wreck of the Titanic.

"They've not only lost a child, they've lost a great opportunity to explore one of the ocean's deep trenches -- the last great frontier for exploration on our planet," he adds. "It's so hard to get funding for deep ocean science, and it took them years to put this expedition together."

Nereus was 30 days into a 40-day expedition to explore the Kermadec Trench when contact was lost under extreme pressures measuring 16,000 lbs. per square inch (psi).

"Extreme exploration of this kind is never without risk, and the unfortunate loss of Nereus only underscores the difficulty of working at such immense depths and pressures," said Larry Madin, director of research at WHOI. "Fortunately, there was no human injury as a consequence of this loss."

In 2012, James Cameron piloted a solo dive to the deepest point in the world's oceans - a record 35,803 feet - in the Deepsea Challenger. He later donated the sub to the WHOI. A 3D movie about the dive will hit theaters in August.

"Perhaps this is an opportunity to evaluate how thin our capability is as a nation, that the loss of a single vehicle now denies us access to an area the size of North America -- which is what the combined area of the deep "hadal" trenches equals. A dark day for many reasons," he writes. "It's sad for me too. I always dreamed of making a joint dive with Nereus and Deepsea Challenger at hadal depth."

Check out a video on the Nereus exploring the Titanic wreck site below.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

James Cameron mourns 'tragic loss' of deep sea sub
The robotic research vehicle 'Nereus' was exploring a trench six miles below the ocean surface when it imploded.