There is a way to kill a mole that is so unimaginably cruel that even an owl might quail before the thought of it. Moles who live in systems plagued by it call it, quite simply, the Talon. ...
It is called a harpoon trap. It has long, sharp prongs set on a spring which are poised above a tunnel in which a pawplate is set. The tunnel is blocked. The mole reopens it, touches the plate and down plunges the unseen Talon, which pierces and squashes at one and the same cruel time. A lucky mole dies at once. But through the paw, or shoulder, or flank, many unlucky ones are impaled, often too shocked even to struggle, and death comes on them with agonized slowness.
— Duncton Wood Part V: The Seventh Stillstone, Chapter 44.
(Pages 526-527 in the 1980 McGraw-Hill hardcover edition.)
Several other mole traps have been used. The "Harpoon" is deadly. It is set off by a plate which is placed above a burrow which has been trodden down, and when the mole reopens the tunnel, the sharp prongs come down with immense force. Sometimes they may spear a non-vital spot, but they usually cause instant death by squashing as well as piercing.
— The Mole by Kenneth Mellanby, published in 1971 by William Collins Sons & Co Ltd.
(Chapter 11: Enemies of the Mole, Page 136.)
The American 'harpoon' trap operates on a different principle [than other mole traps]. Although a murderous instrument, it is easy to set and very efficent. No excavation is necessary, a shallow tunnel merely being pressed down with the foot and the trap placed over it with the trigger-plate resting on the soil surface (Fig. 31). The natural obstruction of earth does not alarm the mole, and when it lifts the soil to reopen its tunnel the trigger releaes the coiled spring causing two sets of spikes to be driven down into the soil with great force.
FIGURE 31. The American 'harpoon trap' shown set over a surface run which has been pressed down with the foot. When the mole lifts the soil two sets of spikes are driven into the ground by the coiled spring
— The Life of the Mole by Gillian Godfrey and Peter Crowcroft, published in 1960 by Museum Press Limited.
(Chapter 9: Keeping Moles—And Getting Rid of Them, pages 130-131.)
Finally there is the relatively rare harpoon, or guillotine, trap...This consists of a framework of flat metal bars which are pronged at the bottom so that they can be thrust into the soil. Across the top of the framework there is a bar through which passes a vertical metal rod, the lower end of which is welded to a plate bearing long and sharp steel spikes. The part of the rod which lies above the horizontal bar is encased in a strong coiled spring. To set the trap the spiked plate is pushed upwards, thus compressing the spring, until it engages against a notch in a hinged catch which acts as a trigger. The major advantage of the harpoon trap is that it requires no excavation of the tunnel and is thus easy to set. Having located a suitable shallow tunnel, the trapper merely presses down a section with his foot and then places the armed trap over the depression with the trigger plate flush on the soil surface. When the mole comes across the depression in the tunnel roof it may try to raise it with its powerful head and shoulders. The trigger plate is activated and releases the spring, thus driving the wicked spikes down into the ground, and through the mole. If the mole is lucky it will be speared through a vital organ, otherwise it will face a slow and agonising death.
— The Natural History of Moles by Martyn L. Gorman and R. David Stone, published in 1990 by Comstock Publishing Associates.
(Chapter 7: Getting rid of a surfeit of moles, Page 104.)
A photo of a harpoon trap, now considered an antique.