Remnants of an ancient Indian pueblo or centuries-old ox deposits? Nobody's sure what lies beneath Santa Fe Plaza in the heart of the nation's oldest capital city, but archaeologists hope to find out.

Watched by plaza regulars, skateboard riders and tourists, an archaeological team on Tuesday began the first of several exploratory digs on the verdant square, which is surrounded by museums, boutiques and gift shops.Within hours, volunteer diggers were unearthing interesting shards of pottery, including a button-size chip of blue and white Mexican majolica clay used in pottery since the 17th century.

Excavation leader David Snow said diggers could find older Plaza levels, Indian artifacts and evidence of frontier life.

"We've already learned something in the top 6 inches," he said.

Santa Fe was the seat of frontier government and a distant outpost of civilization for most of its 380-year history.

The plaza was once a terminus for exploration, trade and immigration routes, including the Camino Real from Mexico City and the Santa Fe Trail from the Mississippi River. It was used for troop encampments, trade markets, for holding oxen from Missouri wagon trains and for growing corn and alfalfa.

"It was the center of the frontier for at least three nations," Snow said. "This was the focus of the frontier up until the railroad came" in the early 1880s.

The plaza is anchored by the Palace of the Governors, the seat of government built by the Spanish starting about 1610. Pueblo Indians threw out the Spanish in 1680, and it is thought that some Indian dwellings and a kiva, a large room often used for religious purposes, were built on the plaza.

The Spaniards returned in 1692. The palace housed Spanish, Mexican and U.S. territorial governors until the late 1800s.

These days, the only conquistadors and mountain men around these days are re-enacting history in the palace, now a museum. Indians sell jewelry to tourists under its portal. Youngsters loiter in the square, riding skateboards and playing games.

Archaeologists will work for about a month on the first phase of the excavation and are restricted to selected areas totaling 10 square meters.

If they uncover enough promising artifacts, and if the City Council approves, a second dig will begin next year under San Francisco Street, the street across from the palace that leads to St. Francis Cathedral.

If that pans out, surrounding buildings are unlikely to be threatened by any excavation.