A Transit of Mars, on Earth?

Well, no, it's impossible, because a Transit of Mars could only be viewed from Jupiter or other body in the outer solar system, as described in this article in Wikipedia.

So why have I brought you to Slough Trading Estate? (I hope you have parked conveniently close to the published coordinates for this listing. If you understand the puzzle you'll be in the countryside for the nice walk soon!)

Indeed, Slough Trading Estate is famously known as the place where Mars Bars are made, and that indeed is the Mars connection to this cache name. So, on to the Transit, which you will be delighted to know is a terrestrial syzygy and therefore perfectly possible and affordable to the average geocacher. No space tourism is required.

Let's make double sure that you are at the right spot: Ipswich Road, at a gap between the light industrial units on the north side of the road, new O2 headquarters to the south, a tree on each corner about you and a CCTV camera for you to wave at. If your GPSr has not quite got you there, you should be within eyeshot and able to get there. Needless to say, don't stand in the road.

The Slough Trading Estate is visible from miles around. Why? Look around you! You're just off the Transit.

Now get yourself onto the Transit which provides the first waypoint, W 000°37.845. Look for an oval aluminium plate bearing a 3 digit number - this is the latitude of the cache final coordinates, i.e. N 51°30.xxx. There is no need to leave the pavement.

Suitable parking for the final location is available at N 51°30.414 W 000°39.780, however, there is no point in going there until you understand the puzzle! Return walk is under 2 miles. Bring your binoculars even if you are not an ornithologist. Enjoy your walk and the cache. It's a stone that isn't a stone.

As a footnote, you will appreciate that the beauty of the cache site is due largely to the abundance of wildlife. Please pay particular attention to the geocaching principle of "leave no trace" at this site and advise me of any adverse circumstances that you may find.