Echo is a Mexican free-tailed bat with a notched left ear and a bad attitude, and he is, according to Riley, a signal amplifier.

She means it literally. When Echo chirps near the green towers at night, some of the chirps don't come back. They go through something: a seam in the air you can only find by the absence of the echo, the way you find a doorway in a dark room by where the wall isn't.

Riley mapped the seams for a year. Most of them are along Shoal Creek. Some are under the Congress bridge, where a million and a half of Echo's cousins pour out at dusk like smoke made of punctuation. A few are somewhere she will not name, off 290, at the mouth of a cave where a woman named Vera Quan gave her last lecture and then, as far as anyone can tell, walked inside and kept listening.

This is what Echo heard on the night of October 30th.

He flew the usual line, bridge to creek, and at the third seam he stopped mid-air and hung there. Bats do not hover. Echo hovered. He sent a chirp into the seam, a long falling one, and this time something came back that was not an echo of the chirp.

It was a voice, very calm, slowed down to bat-hearing and then slowed down again:

What pours out is a way of knowing.

Riley's recorder caught it. Played at ordinary speed it sounds like water dripping in a large room, a measured dripping, with intervals. Played slowly it is Vera's voice, unmistakably, the same sentence she ended her lecture with. Played backward it is the same sentence again.

People asked Riley what she thinks it means. She shrugs, which is what Riley does instead of answering.

But she has started leaving the recorder running at the seams overnight. She says the cave is not empty; it is full the way a held breath is full. She says Vera did not disappear. She went ahead, the way you send someone ahead on a trail to call back what's around the bend.

And on Halloween, when Echo flies, Riley does not follow with the recorder.

She follows with a lantern.