. >> reporter: our guide for this tour of the bracken bat wave north of san antonio is casey andersonwild, on national wild. i love tracking predators. >> reporter: this guy has traveled the world and seen some of the most dangerous animals. and says he has never seen anything like this. >> it is the largest concentration of mammals on the planet. it its overwhelming to be, amongst all these bats. >> reporter: so many bats make the journey together every night. they even show up on the local radar. as many as we are seeing right now, this goes on for hours and hours just a steady stream. >> yeah, this massive flow will go on for three hours sometimes. it just shows you how many bats are in that, in that cave. it's -- it's, blows my mind. >> reporter: as close as we are here, you are telling me i am in no danger if one hit me in the head? >> no, only thing you are in danger of. you might feel look you are getting rained on. that's not rain. >> that's danger to me, let's be very clear we couldn't go in the cave from the dangerous level of c 02 from the bat's guano. casey did go inside a

anderson? >> we'll see how he is tonight. more on the legislative side of things. i spoke with bob casey, democrat of pennsylvania.or, this alternative resolution you're working on, as i understand it, it would require passage of the u.n. security counsel of assad using chemical weapons ex seat a deadline to get rid of the stockpile and a military strike would be authorized if that deadline isn't met. what can you tell us? >> we're trying to do everything possible to make sure first and formost that the regime in syria and the russians know that we're going to continue to pressure them to come to a resolution of this, but i think there is enough consensus to develop a strategy where we might be able to get an even better result, meaning all chemical weapons out of syria without fail. so that means the syrians and really the ball in the court of the syrian regime and the russian federation -- >> in terms of this resolution, though, putin, vladimir putin said today that this can only work if the u.s. and other countries renounce a plan to use force against syria. is that force continuing essential? >> i think it's c