Dive summary:

• In November 2011, a boring machine that workers call Harriet began to push down into the limestone beneath Miami Harbor from Watson Island, and on May 6 it finished a 1.6-mile round trip.
• The twin 4,200-foot tunnels that Harriet chewed between Watson Island and Dodge Island will link the port directly to Interstates 95 and 395 for truck traffic.
• Bouygues Civil Works Florid, the design-build contractor, now has a little over a year to do the rest of the work needed to commission the tunnels.

From the article:

The design-build project – which is being delivered via a public-private partnership – marks the first time that a large-diameter tunnel has been bored through coralline limestone, which is highly porous and unstable. ...