Crawling in small spaces- instinctive behavior?

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Let's play tug!!
Barked: Mon Nov 5, '12 1:08am PST 
Smokey generally has a good recall off leash. He will usually even come if he's in the middle of playing with another dog, or sniffing a dead thing. However, when he sees large rocks, he seems to become possessed. He will take off sprinting down the beach and put himself in great danger. He has climbed up rocks five times his height and started walking out to sea, climbed up moss-covered slopes and slipped and slid, coming very close to falling, and wedged himself in tiny coves as the tide came out, and then screamed for help as they begun to fill with water. Then I climb up to get him out, heart pounding, and fervently hoping nothing will happen until I get to him. I have to keep hold of his harness or he will go back to climbing as soon as he is out of direct harm. He has scared me enough that I now don't let him off leash until we're well past the rocks. It seems to be a kind of over threshold- his eyes glaze over and he becomes totally focused- although his body language when he's doing it is generally confident (which is extraordinarily unusual for him) and almost prowling. We recently visited a place with safer large rocks- they were back on the shore rather than right on the sea line, and he explored for hours, even being more interested in the caves than dogs or treats. Since he also crawls under the bed, burrows in the covers, hides treats under sheets, blankets, laundry, etc, I'm curious whether this is an instinctive thing. He is half Frenchie, and I think I remember hearing that they burrow. Could it be a prey thing, looking for small animals? I'm curious whether anyone else has seen this or has any ideas what it's about.