In yesterday’s post, the glider team took a calculated risk and brought glider RU05 into the shallow waters near shore. We made a big deal about how dangerous this was—how the waters near shore are full of obstacles like rocks, islands, icebergs, and squirrelly currents. But we also said that everything went fine and the glider team was already planning more. If it sounded like maybe shallow water wasn’t so dangerous to gliders after all, then today’s post is for you.

Late yesterday evening, glider RU05 dove to the bottom of the sea as usual, but it never came up. When Dr. Josh Kohut woke up this morning, he had a text from the glider pilots in New Jersey: RU05 has not checked in for more than 8 hours. Something was keeping it underwater.

We spent the rest of the day figuring out what had gone wrong. Click through the slideshow to see what happened to the glider and how the team fixed it:

This evening, glider pilot Dave Aragon looked at the glider’s data files and made this schematic showing what he thinks happened. The glider was doing its normal job, repeatedly diving to the bottom and rising to the surface, and measuring the water along the way. The zigzag black line shows the glider’s actual path. Aragon thinks the glider did fine but may have flown into a patch of kelp (a type of very large seaweed). The thick kelp stems got tangled in the glider and didn’t let it go until 12 hours later, when it dropped its emergency weight.

This is one of the hard parts of working with gliders. They may be sophisticated machines, but they can’t see in front of them, they have limited battery life and limited maneuverability, and once they go below the surface they can’t contact the glider pilots for help. If it gets caught underwater, there’s a very real chance the scientists will never see it again. Today, we were lucky.

“Off New Jersey the shallows don’t really present a danger to the gliders,” Dr. Kohut said. “Here with all the kelp and the rock it’s much more hazardous. But I’m still happy we sent the glider in close to shore because we got all that very nice data to go along with Kim [Bernard]’s transect.” From now on they will be a little more careful around shallow water, though. “Before this, we knew we needed to avoid islands,” he said. “Now we’ve expanded our definition of an island to include water up to 20 meters deep.”