Forty
About 50 firefighters are battling a blaze on Ministry of Defence land near Colchester.
The Middlewick Ranges on Mersea Road in Colchester is also used by local people. It is home to nightingales and other wildlife.
Essex County Fire and Rescue Service has asked the public to avoid the area so crews can park fire engines near hydrants.
Mike Lilley, a Labour city councillor, said: "Residents are getting hoses and buckets of sand ready to protect our homes."
Resident Aimme Keteca, 41, said: "It looks pretty serious, the black smoke is substantial."
John Hood saw smoke billowing "hundreds and hundreds of feet up in the air" from where he was at Colchester United's football stadium 3.7 miles (6km) away.
Brian Gilbert told BBC Essex presenter Ben Fryer that he could see it from where he was at Abberton Reservoir
"It looks very, very big," said the 68-year-old.
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