Rain falling on the surface of the ocean can be heard more than 1.2 miles down in the water, and at some sound frequencies, it's louder than passing ships, according to oceanographer Jeff Nystuen.

Nystuen, of the Applied Physics Laboratory at the University of Washington, is among a number of scientists studying the ways in which sounds travel through ocean waters to better understand how loud, man-made noises might affect marine life.

"We don't really know what is too loud underwater and we need to know what the baseline is," he said.

Loud underwater noises, particularly Navy sonar, have long been blamed by environmentalists for the fatal beachings of whales.

The Natural Resources Defense Council and other plaintiffs have alleged in a federal lawsuit that the Navy's mid-frequency sonar used for detecting enemy submarines disturbs and sometimes kills whales and dolphins.

The Navy settled a similar lawsuit three years ago by agreeing to limit its peacetime use of experimental low-frequency sonar.

To find out what sounds sea creatures are exposed to, Nystuen moored microphones at spots around the world from the Mediterranean to the South China Sea to collect a year's worth of sound passing through the waters. He spoke Thursday in Honolulu during the biannual ocean sciences meeting of the American Geophysical Union.

Building up his inventory of sounds since 1999, Nystuen is looking for long-term patterns of relative loudness.

His findings show that among higher-pitched sounds, the loudest 1 percent are dominated by the noise of rain, at 94 percent, followed by ships at 3 percent. But among lower-pitched sounds, shipping makes up 87 percent of the loudest sounds, followed by rain at 9 percent.

Nystuen's recordings haven't yet been able to account for the erratic behavior of marine life.

"If you came to see me in Seattle and said, 'I want to see some killer whales,' I would take you to Haro Strait, which is the noisiest environment that I've ever made measurements in," he said. Haro Strait lies between the U.S. and Canada near Vancouver.

Nystuen said his listening devices could be used to make sure marine animals aren't nearby before setting off a blast or testing sonar.

The U.S. Army Corps of Engineers spent three years figuring out how best not to harm protected species during a project completed last summer at the port of Miami that required the agency to blast solid rock out of the harbor, said Terri Jordan, a biologist with the corps.

"Sound and its associated pressure have obviously been shown to have adverse effects on marine animals," she said.

The techniques used on the project included capping and staggering blasts to minimize the sounds.

But before triggering any explosion, a 4,000-foot perimeter was searched by a helicopter and two boats for signs of any animals, including porpoises, manatees and sea turtles.

If any animal wandered within about 2,500 feet of the blast site, the project stopped until the animal wandered out again.

"Our big-time holder-upper was turtles," she said.

The perimeter's width was calculated by doubling the distance usually used to protect Navy divers from being hurt by the sound and pressure of bomb blasts. Five minutes before the main explosion on the project, a smaller "fish scare" blast was also used to scatter any fish in the area.

During the 40-day project, workers logged 186 animals and only one possible reaction — a nearby dolphin that jumped out of the water during a blast.

As shipping traffic increases, ports will need to expand their capacity and often will need to chop into rock to do that, Jordan said.

"The only way to crack that rock with the current technology that's available to us is the use of blasting," she said. "So we think this is where we'll be going in the future unless new construction techniques become available."