SACRAMENTO, Calif. - What looks like a simple hole in the ground with rotting wood inside could also be a portal to American history.

Railroad buffs believe a sinkhole at a construction site in Old Sacramento in California has led them to fragments of a trestle bridge from the first railroad line to span North America.

"We're about 80 to 90 percent sure that this is a piece of the transcontinental railroad of the Central Pacific," said Kyle Wyatt, curator of history and technology at the California State Railroad Museum.

There is no way to be certain, Wyatt said, but circumstantial evidence is strong.

The find's location matches old maps that show railroad tracks crossing China Slough, a waterway that once drained into the Sacramento River.

The wooden beams clearly form part of a very old bridge, its once straight-hewn sides decayed away to expose smaller branches jutting out.

There was also a wagon bridge nearby, according to maps, but Wyatt believes the close spacing of supports and heavy construction of this bridge make it likely it was built for railcars in the 1860s.

If so, it wasn't used for long. By 1879, the railroad had been rerouted so trains could cross the river on a large ferry. By the 1880s, China Slough, also known as Sutter Lake, had been filled in and Sacramento was growing up over it.

"The railroad museum is built on top of a former swamp," said museum director Paul Hammond.

Sacramento's history of filling as it went has covered other lakes and bridges, including a trestle found a few years ago during excavation for the Seventh Street underpass through the railyard area.

"It was perfectly preserved," a single-line trestle built of redwood, said archaeologist Kim Tremaine, who helped document that find before it had to be torn out.

This discovery will be probed as archaeologists and railroad fans expand the hole southward, looking for more bridge supports.

They'll take photos and measurements, but ultimately will cover everything up again.

"It would be problematic to preserve it as an open site for display," Wyatt said, because the location is low and close to a levee.

Instead, the museum expects to go ahead with its original plans for the site, which lies near the I Street bridge approach. The museum will extend some track and pave the surrounding area to create a maintenance space for railroad cars.

Early stages of that work, which involved scraping off some surface soil, led to the trestle discovery. When the exposed land was pounded by recent storms, rainwater drained unevenly around the rotten wood and created the sinkhole.

There are plenty of places where people can spot old railroad grades or signs of old tracks, Tremaine said. It's less common to find buried remnants of a bridge.

"You don't think about what's underneath the ground, and there's a lot, especially in Sacramento," she said.