L.A. is a city with a skeleton of concrete. For graffiti artists, this corpse can be a canvas. In some urban corners, often left untouched (and unbuffed) by the powers that be, graffiti murals spring up like trees planted by invisible arborists. But these areas, like old-growth forests, are becoming rare. The mouth of the Belmont Tunnel ? where the city?s first subway ran ? was once the premier guerrilla gallery for urban artists. Today, lofts stand in its place, but one classic, old-school graffiti spot remains: the bridge at the confluence of the Arroyo Seco and the Los Angeles River. Murals, scrawled in the imaginative typography of the street, span the gray walls, sometimes extending down into the green waters of the basin. And on the undersides of the bridges, markings of the original graffiti artists, early-20th-century hobos, are written in coal swiped from the engine rooms of freight trains. Past meets present, forever etched on the urban walls. Under the Arroyo Seco Parkway, in the L.A. River. ?Drew Tewksbury