JIM LEHRER: Finally tonight, why some Japanese cities were destroyed by the tsunami, despite seawalls and other defenses.
Tonight's "NOVA" program looks at the science behind the March 11 earthquake and tsunami, and why one town, Miyako, was so hard-hit.
The documentary includes footage from a conservationist who was there at the time.
Here's an excerpt.
NARRATOR: This area has good tsunami defenses. The residents are well-prepared. They should be safe.
Last time a tsunami hit here was half-a-century ago. In the aftermath of that tsunami, they built these 30-foot-high seawalls. Tsunami drills are a regular feature of life. Everyone knows what to do when the sirens sound.
MAN: Yes, we're leaving? Where do we go?
MAN: Go to the hill.
NARRATOR: On March 11, the siren sounds. The tsunami is minutes away.
MAN: There's a hill outside of town that we're going try to get to.
NARRATOR: Confusion rules. Some search for high ground. Some aren't sure what to do.
MAN: Well, it's a precautionary measure, but, I mean, you never know. This side of town has a lot of history with tsunamis and a lot of death from it. So they're taking it pretty seriously, obviously.
NARRATOR: The warning saves the lives of some.
MAN: Here it comes!
NARRATOR: The tsunami easily breaches the coastal defenses. Miyako's high walls are useless. The tsunami is 30-feet-high. So, why did the 30-foot-high walls fail? Data from thousands of sensors along the coastline suggest a stunning answer.
ROGER BILHAM, University of Colorado, Boulder: The fact that the shoreline has actually subsided means that the sea had plenty of space to go. And it -- it basically filled up the empty space left by the sinking. Several villages have just been completely ruined, with no survivors. And the human death toll is obviously going to be up in the tens of thousands when -- when the final count is in.
NARRATOR: The earthquake causes the whole coastline to drop by up to three feet, lowering Miyako's walls and making the tsunami much worse.
JIM LEHRER: The official death toll from the tsunami and quake now stands at more than 11,000. That number is likely to grow to nearly 20,000.
"NOVA" airs on most PBS stations tonight.