At the time of publication this will be the fourth set of Adventure Labs following the course of the Greenwich Meridian across London. This set of five centre on the point where the Meridian Line crosses the River Thames.
North/south Meridians can be drawn pretty much anywhere you like on a map. London is significant because it is passed through by the first internationally agreed Meridian at Greenwich; since 1884 the 'Prime Meridian of the World'. This line remains, give or take a few metres either side, the basis for mapping and navigation today.
There is no container at the posted coordinates. You do not need to enter the River Thames to find this cache. There is no need to get wet. Instead you will need to complete the five stages of the associated Adventure Lab series. After completion of each stage you will receive a piece of information which, when complete, will give you the coordinates of the cache and a hint to help you find it.
The container at the final location is expected to be a simple find at an accessible location. The difficulty level and terrain rating reflect the work that you will have to put in to determine the correct coordinates.
An extra hint now for those of you who read to the end of the description section. You will be well advised to complete the Adventure Lab series from south to north if you intend to look for the Bonus at the end.