The Big Dig

There is an impressive hole in our garden, so deep you cannot see the ground (and certainly no fence to prevent you from falling down). What is this for? Oil? Diamonds? A secret connection to the Russian embassy?

Someone tells me it's to prevent the water form the roof from destroying the street. Aha... collecting rainy water instead of draining it onto the street.

But the puzzle remains: Why should the water flow into the hole instead of out onto the street? And why should our house's owner care about the street while others don't? Why not collect money from all house owners and build a ditch?

Update 2007-11-26:
This hole is at least 12m deep (the string with a weight on one end was 12m long and could not reach the ground). We also noticed steps carved into the wall, indicating that the digger climbed down and up without any help.