Deep trenches, unexplored surfaces and a long, treacherous journey: Sounds like a voyage to Mars, but it also describes the trip to the bottoms of Earth's own oceans, about which we know even less than the Red Planet.

What better way to explore the unknown than with an array of robotic subs? Human-occupied underwater vehicles are limited by how long they can stay underwater, and those that are unmanned traditionally have had to be connected to ships via a long control cable. Due to advances in programming and communication, some have been able to go cordless.

Earlier this year, reported on a few of these submersibles. Here’s an update on where a couple free-ranging subs are now.

One independent swimmer, Nereus, has now reached the Pacific's Mariana Trench, the deepest part of the ocean, reports the BBC. It's the first time in more than 10 years that a vehicle has ventured into the trench, and a dive last Sunday took the sub down to a depth of more than 35,761 feet (10,900 meters).

"With a robot like Nereus, we can now explore virtually anywhere in the ocean," Andy Bowen, principal developer of the sub at the Woods Hole Oceanographic Institution, told the BBC. "The trenches are virtually unexplored, and I am absolutely certain Nereus will enable new discoveries."

Another autonomous oceangoing vehicle was launched off the coast of New Jersey in April. Rutgers University's RU27—aka The Scarlet Knight, after the school's mascot—is gliding across the Atlantic via the Gulf Stream current, measuring temperature, salinity and water density along the way, reports Wired. Notes from the research team reveal that the glider craft is making good time and even comes to the surface from time to time to check in (via remote communication) with the researchers back home—calls they wait anxiously for.

View a slide show about mini subs.

Image courtesty of Woods Hole Oceanographic Institution