With almost glacial slowness, Metro construction workers yesterday began delicately sinking the first of three giant subway tunnel sections into the Washington Channel - another link in the long-planned "second cross" of the Potomac River by Metro.

The painstaking engineering feat should be completed sometime tomorrow when the 340-foot long prefabricated steel section comes to rest in a specially dug trench 60 feet below the channel's surface.

Workmen pumped tons of concrete into ballast boxes along the sides of the floating section yesterday, forcing it to sink slowly into the muddy channel water. Four winches with cables mounted on huge barges held the section in place, keeping it level and regulating its slow descent.

"It's a simple positive method of putting in a tube," said resident engineer Ted Brayman as he led a group of reporters and photographers on a tour among the maze of cranes, cables, ladders, bollards and steel girders surrounding the tunnel section.

The 1,200-ton tube - one of three manufactured at Port Deposit, Md., and floated down the Chesapeake Bay and up the Potomac estuary to the Washington Channel last year - is plugged at both ends with steel bulkheads. It bobbed on the water like a gargantuan metal cork.

Brayman said the submersion must be slow and precise to ensure that the section comes to rest in exactly the right position - on the west side of the channel adjacent to the Hains Point peninsula in Southwest Washington.

A second tunnel section will be sunk and positioned on the east side of the channel in September, Brayman said, and the final, middle section connecting the other two will be put in place in October.

The sections will be sealed with special underwater concrete and the bulkheads removed so that workmen can enter and start laying track, Metro spokesman Cody Pfanstiehl said the line will not be opened for passenger service for at least three years.

Once completed, the tunnel will be part of Metro's Yellow Line connecting L'Enfant Plaza Station at 7th and D Streets SW with the Pentagon Station in suburban Virginia. This will provide a second crossing of the Potomac. The Blue Line currently connects downtown Washington with Rosslyn in Arlington.