(NewsNation) — After the container ship Dali crashed into a pillar of the Francis Scott Key Bridge in Baltimore, causing a bridge collapse, the cause of the accident is still unknown. The ship had two harbor pilots on board, a critical role for ships heading into port.

What is a harbor pilot?

Harbor pilots are experienced maritime professionals who help guide large ships into port. Pilots are required to be experts in the specific port where they work and are responsible for steering vessels the size of a skyscraper weighing thousands of tons from the open water to the pier where they will dock.

Pilots approach a container ship on a small speedboat, where they climb a ladder up the side of the ship to pilot it into the port. After guiding ships back out of the harbor, pilots again descend a ladder that can be as long as 40 feet before leaping back onto the speed boat.

Why do ships use pilots?

Using pilots is required by law, with legislation dating back to the 1700s in some states. They direct the ship’s crew to maneuver the vessel in areas where there may be only a few feet of clearance, something that can lead to ships running aground, as the Ever Given did in the Suez Canal.

Harbor pilot qualifications vary by state but generally, they are required to have multiple years of experience and go through exams and testing to prove their familiarity with the local area. In Maryland, pilots must be licensed masters for large vessels with at least five years of experience as well as passing a state exam.

Pilots also play a national security role. With many foreign-flagged ships coming into U.S. ports, pilots may be the only American crew on board who might notice anything suspicious. In Maryland, 96% of the ships coming into port are helmed by foreign crew.

What are the risks of the job?

Pilot is one of the most dangerous jobs in the maritime industry, with pilots facing a one in 20 chance of dying on the job. The biggest risks come as they must jump from the speedboat to the container ship and back. Both vessels are moving and strong currents or winds can toss the smaller speedboat around, making it a moving target.

Pilots risk falling in the water, getting run over by the large vessels and getting trapped between the speedboat and container ship. The risk does come with a reward, as the role is also one of the most highly paid, with some pilots making upwards of $400,000 per year.

Why do accidents still happen?

While pilots are experts at steering ships into port, they aren’t immune to accidents. If a ship loses power or suffers a mechanical failure and is moving too fast to drop anchor, there’s little a pilot can do to prevent an accident. Similarly, bad weather or poor visibility can lead to accidents even with skilled pilots aboard.

The cause of the Francis Scott Key Bridge collapse is still under investigation by the National Transportation Safety Board.