There must be some unusual chemistry in the mud.
Maybe its been frozen solid for 60 years & has now melted. Given there is grass growing there must have been oxygen in the mud. Even if there was no oxygen, inoxic or anaerobic bacteria would have made a meal of it. There are plenty of iron fixing bacteria that in 60 years would have digested it.
Being frozen is the most likely reason for preservation if it has been there any length of time.
Or it has not been there for 60 years. After that time no one would know it was there.
My guess is it was driven in there by some rebels & has been recovered. The tracks should have been seized, that gun would not have been able to be moved. the paint would not sill be on it, electrolysis would be every where.
Would have Probably only been in there a week !!

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving