The underground reservoir is covered in vegetation
Residents fighting plans by Thames Water to sell a Berkshire reservoir to allow 100 new homes and offices to be built have held a public meeting.
Save the Bath Road Reservoir Group says the 5.4 acre (2.2 hectare) underground site, which is covered in vegetation, is a "green lung" in central Reading.
It held a meeting on Wednesday night to urge residents to take part in a public consultation, which begins on Thursday.
Thames Water said sale profits would be reinvested to improve water services.
The company said it had a duty to customers to reap the maximum value from selling the land, adding it was worth far more with planning permission.
It said residents' opinions from the public consultation would help shape its plans, to be put forward to Reading Borough Council later in 2009.
However, the opposition group said the loss of the 150-year-old site would impact on wildlife.
Although the reservoir is not open to the public, the vegetation has been untouched for decades allowing wildlife such as deer, foxes and badgers to flourish, it said.
It wants Thames Water to turn the area into an educational resource for local schools.
Hundreds of residents attended Wednesday night's public meeting
The group said its "sustained pressure" had already forced Thames Water to withdraw earlier plans for developing the reservoir last year.
Phil Birch, a campaigner with the opposition group, said: "It's about finding appropriate sites.
"There are lots of vacant sites in and around Reading that could be utilised and could be more preferable."
Mel Woodward, also from the group, added: "We won a significant first round against Thames Water last year when they withdrew their earlier planning application but we will need a real effort from the whole community to see off their latest plans."
Thames Water's plans are on display on Thursday and Saturday at All Saints Church Hall in Downshire Square.