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OP here
I went back with a friend and a shovel.
Cleared the floor so the crawling was smoother.
Went deeper in.
Got to the point in the picture, maybe about 12 to 15 meters (40-50 ft) or so, then got too scared.
The tunnel goes at least another 15 meters but at that point it was quite tight. It would still be possible to crawl the whole way.