A C Highfield 
Outdoor (Natural) Hibernation
Definition:
Outdoor or natural hibernation consists of allowing the tortoise to excavate its own burrow instead of hibernating it in a box under controlled conditions. In the wild tortoises usually dig themselves in under large rocks, tree roots, or into the side of earthen ledges. Excavations may be several feet deep. Some species, such as Testudo horsfieldi, excavate burrows that are several meters long.
In captivity:
Natural hibernation offers some advantages, and some disadvantages. In its favour, freezing is very unlikely to occur - even under severe weather conditions - as at anything beyond a few inches underground temperatures are very stable indeed. Even in the most severe weather, frosts rarely penetrate more than 2 inches deep.
The disadvantages of the method, however, include:-
Do not allow tortoises to burrow in an area where flooding is possible Perform extra-careful health checks throughout the summer and especially during the period immediately prior to hibernation! The area above a hibernating tortoise can be protected by covering the ground with wire mesh - but be watch out in warm weather as the tortoise may be trying to emerge!
References:
Safer Hibernation & Your Tortoise (1986-1992), Tortoise Trust (Free on Request)
Safer Hibernation & Your Tortoise - Video Edition £9.95 inc. P&P
If your tortoise has safely hibernated itself in this way before, then there is no necessity to vary its routine. Natural hibernation is not intrinsically dangerous.
Bearing these possible problem areas in mind, you can do much to minimise the dangers.
All of the other information given in "Safer Hibernation & Your Tortoise" applies equally to natural hibernation. The health checks described in the booklet and shown on the new video are particularly important, however.