A rumble sounds on the African savannah.

The matriarch speaks, urging the herd to leave the watering hole.

But the message reaches more than her own kin.

Other elephants, too far off for the sound to reach their ears, get the message, too... ...with their feet.

A half hour's march away, a rival matriarch directs her own clan to the watering hole now that she knows it's free.

A roving, solitary bull starts to move towards the matriarch's call too, hoping to find a mate.

Scientists call this seismic communication.

It's how the world's largest land animals keep tabs on each other in a place where eyes and ears aren't always enough.

With vocal chords eight times the size of ours, elephants send out two types of sound waves at once.

There are the high frequency ones.

Those travel quickly through the air and land on the ear.

They only go a short distance, about a mile or so.

The second are low-frequency waves.

They move through the earth, and travel much further -- at least 5 or 6 miles.

When elephants sense one of these low rumbles, they halt, and press their heavily-padded feet to the ground.

That padding, a huge ball of fat called a digital cushion, spreads out as much as 20 percent.

Dozens of touch receptors in the elephant's foot, called Pacinian corpuscles, pick up these vibrations and send a signal to brain.

But hold up, is that hearing or is it feeling?

Well, turns out the line between those two senses is pretty fuzzy.

Scientists think ground vibrations also travel through the elephant's skeleton, all the way to the ear.

That's how feeling becomes hearing.

Caitlin O'Connell researches hearing at the Stanford University School of Medicine.

In a series of experiments in Namibia, she buried speakers in the ground.

Then she played common elephant calls, to see what would happen.

When this herd hears an alarm call played from an above-ground speaker, it means danger is close, they retreat.

But when the same call is played underground, they react more slowly.

They grow wary and close ranks -- but stay put.

When the elephants get the message through their feet only, it means the danger is far away.

Even an elephant's footsteps transmit useful information about its size and whereabouts to other elephants.

So if you know how to listen, this dramatic landscape hums with signals, and a web of sound holds a whole community together.