When the Nathan Marsh Pusey Underground Library opened its doors this week, some of its system-builders were there admiring their handiwork.

There are plenty of systems to admire. For one, since the new library is sunk well down into the water table, it sports what the planners call a "triple-level waterproofing system"--complete with its own perimeter drainage system backed up by diesel generators.

The Pusey Library is also the world's first to be designed with a built-in, fully-automated, halon-gas fire-extinguishing system.

Activated by a special "Pyr-a-larm," halon gas pours out of sinister-looking outlets in the ceiling and puts out fires instantly.

"The beauty of halon gas," Robert R. Walsh '65, assistant University librarian for building planning, said yesterday, is that it doesn't damage the books, as water would, and it doesn't suffocate people, like the carbon dioxide in Yale's Beinecke rare books library.

"Halon reacts with the molecular process of combustion and makes it absolutely impossible for that process to continue," Walsh explained.

Right now, the Pusey Library doesn't have many books to burn. The social sciences collections have only begun to be moved in from the Widener stacks into the subbasement of Pusey, and the theater and map divisions on the main floor are in the last stages of getting organized.

Only the archives--with shelves of Harvard historical papers, year-books, anniversary reports, and course catalogues dating to 1803--had gotten into full swing.

This week, most of Pusey's visitors have only been browsing--"touring around to see what's what," as one grey-haired woman said yesterday--and admiring the orange-and-white decor and the asymmetrical passageways.

The views from the outside windows are generally bleak, since they tend to be dominated by the walls of the pit in which the library is sunk.

But a student who works in the basement of Lamont said that the design is very nice--"one of the main things I'm impressed about is the light they're able to get in here."

Robert Walsh considers the Pusey Library to be a pedestrian triumph, too. There will be no guards at the doors challenging passers-by, he said; the guards will be posted at the entrances to the individual collections.

"This way the library becomes a continuation of the pathway system of the Yard," Walsh said.