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Large heaps of earth appear on the lawn. Long raised or sunken tunnels can sometimes be seen.
Moles.
A lawn can be completely ruined.
Organic - it is best to try and prevent moles from setting-up residence under your lawn in the first place. Getting rid of them once they have arrived is very difficult. There are sonic deterrents which emit very high-pitched noise that the moles should find unbearable. In reality these have variable success rates. An old wives' tale that is worth trying is to stick children's windmills in the lawn – the moles don't like the low humming noise they make. Similarly, empty milk bottles buried in the garden, sticking out by 15cm (6in), also make a low hum as the wind passes over them.
Chemical - there are traps to catch the moles and gas to kill them. However, these treatments are best carried out by professionals.