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President Trump made building a border wall between the U.S. and Mexico a cornerstone of his 2016 presidential campaign. But when, after the election, efforts to build the wall stalled, he turned to other possible options — including constructing a trench filled with snakes and alligators — according to a forthcoming book.

"He would raise this idea of a trench — and [that] maybe we could have a water-filled trench," New York Times journalist Julie Hirschfeld Davis told Fresh Air on Wednesday. "And he raised [the idea] so many times that actually his aides finally went and got a cost estimate for what a trench would cost." Davis notes that thoughts of a border trench were cast aside after it was estimated that it would cost three times as much as a wall.

Davis and her Times colleague Michael Shear have covered the Trump administration from its earliest days. They chronicle the president's attempts to upend the nation's immigration system in their new book, Border Wars: Inside Trump's Assault on Immigration, publishing on Oct. 8.

According to Davis and Shear, Trump sees a border wall as something more than a physical barrier. "The wall, in his view, should not just be a structure that would stop people on the other side, but that anybody who tried to climb it would be hurt severely," Shear says.

To that end, they say, the president has proposed electrifying the wall or topping it with spikes and sharp concertina wire that would cut anyone who tried to climb it. He once suggested to now former Homeland Security Secretary Kirstjen Nielsen that the wall be painted a "flat black" that would heat up in the sun and burn anyone who touched it.

"At one point, he actually publicly said that when migrants would throw rocks at the military or the border guards, that the military should just respond with rifles, implying that they should just kill them in response," Shear told Fresh Air.

Shear notes that when aides told Trump that they could not respond to rock throwing with lethal force, the president backed down — but only slightly.

"He suggested, 'Well, why don't we just shoot them in the leg? That'll slow them down,' " Shear says. "The aides sort of sat there kind of slack jawed and finally said to him, 'No, sir, you can't do that either.' "