you basically have no light, down to about half a mile to a mile, you have some diffuse light, but sea creatures often generate their own life. it's called biolum -- for example when the submarines are slipping along, and they pick up their transmitted from a tower. when they do send stuff down there, they have to be mindful of these kind of density gradients in the water. >> thank you so much, i think we learned a lot right here. >> it's interesting because the blue fin submersible, it doesn't need to see, per se, it can do a sonar map right there. using different types of technology. >> every time they do one of these searches, they learn more