The crew of the survey vessel Catlett are no strangers to navigating the channels around the Port of Baltimore. Outside the boat’s tinted windows is a constant reminder of why their most recent survey trips are different than normal. Pieces of the Francis Scott Key Bridge jut into the air alongside cranes and shipping containers around the bow of the Dali.

As a part of the U.S. Army Corps of Engineers, the crew’s job is to slowly navigate back and forth in the water — like a lawn mower — so their instruments can create a hydrographic map of what’s under the surface. A hydrographic map,more commonly known as a marine map, shows the contours of the bottom of the coastline, the depth of the water, and the tides and currents in a given area.

The crew’s current mission is to look for debris from the Key Bridge collapse. They are one part of the complex operation playing out in the Patapsco River to reopen the port.