After finishing my coolant flush and re-mounting the two belly pans I found to my shock a big bolt on the garage floor under the engine area. The hex bolt is about 1 1/2 inches with the head 1/2 inch in diameter and thread diameter is 1/4 inch as in the picture.

When I found it the car was still on jack stands and I was lying on my back under the car, so that no other action could introduce it from elsewhere.

I certainly did not touch such a bolt during my coolant R&R and I did not hear or see it fall down. My infinite worry is where it's come from. Such a big bolt when lost is no minor matter.
I'd appreciate if you can identify it. Thanks a lot!

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE