Concrete?
Nah. Just use seawater.
Of course, I guess they could still scuba through.
Maybe fill 'em with aggressive, poisonous critters.
Or fill 'em with traps ... like an Indiana Jones obstacle course.
I know.
It just seems so ... boring.
OH! I got it!
Take the radioactive waste from the tunnels at Hanford, and use that along with the cement.
Sort of like the solution McArthur proposed to keep the Chinese out of Korea.
Solve two problems at the same time.