(Story photo - Photographer Robert Martore; Courtesy South Carolina Department of Natural Resources)

It was the final trip for thousands of New York subway cars: down the Harlem River, through the harbor and out into the ocean.

Last stop: the bottom of the sea. 

"There's a lot of emotion to these cars. I've been on them, I've held them, I've waited for them. Amazing experience for me seeing them thrown into the ocean," photographer Stephen Mallon said.  

Photographer Stephen Mallon was scouting a photo shoot when he saw a barge stacked with old subway cars. He had stumbled on a program that dumped the cars off the East Coast to create artificial reefs.

Mallon decided to document the process. 

"I would ask the captain to put me in certain places to capture the lighting and the energy of what was happening," he said.  

The New York Transit Museum is displaying Mallon's work, an exhibit called "Sea Train: Subway Reef Photos."

Curator Amy Hausmann says she was fascinated by his photographs. 

"These are all subway cars that we know, that we rode, we're familiar with them, we touched them, we were passengers on these cars like millions of other people over the course of 40 years. Now, these trains have a new life under the water," Hausmann said. 

Mallon spent 2008 to 2010 photographing the disposal program, following the barges out into the Atlantic and capturing the moment when these giant pieces of machinery were dropped into the abyss. 

"They're in midair for a moment and there is nothing happening, and then all of a sudden, there's this splash, and so, you know, it was the largest splash photography shoot I have ever done in my career," Mallon said.  

The reef program began in 2001, at a time the MTA had an unusually large number of surplus cars. The carriages were stripped of all toxic materials before they were dumped. 

It was a win win, saving the MTA $30 million in disposal costs and helping marine life offshore from New Jersey to Georgia. 

"They have created this new biodiverse habitat for sea life," Hausmann said.  

The disposals ended nine years ago, after all the old cards were dumped. But the artificial reefs endure, helping species from mackerel and flounder, to mussels, barnacles and coral.

The glimpse of the underwater world of New York City transit will be open to the public until June 16.