JUNEAU - The Alaska Marine Highway System is trying to figure out how 50 to 60 gallons of sea water got into a fuel tank on the ferry Tustumena.
Initial inspections by the Coast Guard and dive teams last week couldn't find a leak, and no fuel is escaping.
It's a mystery the system hopes to unravel in a Ketchikan drydock, where the ferry is expected to arrive Saturday for an emergency investigation.
The 174-passenger ferry, built in 1964, is out of service indefinitely until the problem is identified and fixed.
The 499-passenger Kennicott ferry will provide substitute service.
A department spokesman says he doesn't know how many passengers are affected, but that reservation holders will be accommodated.