Follow this link :
http://www.wonderfulinfo.com/islam/zamzam/

The Zamzam well was excavated by hand, and is about 30 metres deep and 1.08 to 2.66 metres in diameter.

It started 4000 yrs ago.

Zamzam water has no colour or smell, but it has a distinct taste, and its pH is 7.9-8.0, indicating that it is alkaline to some extent.

Never dried since then.

Never changed the taste, no Algea or plant growth in the pond, thus no disease caused.