What location options are available for a large-scale underground mining city

Question

In the late 18th century, a strange project was proposed by a Dutch engineer regarding a certain English expedition. The British Empire held a vast amount of territories at this time and had much to protect. Certain members of the Royal Navy discussed whether it would be feasible to start and maintain a large-scale colony (Around 20,000 or more) in one of their outer territories to serve as a base for a military outpost. Britain had started many colonies before. However, this one was to remain utterly secret because of its military potential.

The purpose of this idea was two-fold: Operate a self-sustaining colony that can provide Britain with resources. And allow them to have a distant military outpost to protect their vast empire. One can operate in secret, including if necessary, from underground should the mainland or important territories fall under attack.

Not only would this colony have to be secret, but also able to withstand attacks and be well defended.

The engineer in question suggested an underground city or fortification of sorts. He was inspired by the underground networks and cities found in ancient cultures around the world. The man figured with the "modern" equipment and the vast resources at hand, the Commonwealth could surely utilize something similar.

The project began in the 1890s but picked up the pace from the early 1900s to before the war. By then, interest in the secret project had declined. Its high cost and the threat of war caused attention to be diverted elsewhere. Even though this outpost was designed to serve a military purpose, the Homeland and nearest territories were deemed of higher interest. And overtime the mining colony whose true purpose is only known to a few was abandoned.

As time goes on, the project was eventually disbanded, though most of the residents still remained long after they were abandoned. Being able to utilize the self-sufficient capabilities of the colony.

This develops into the main plot of the story.

My two questions are:

1. What remote geographical location would serve best for this underground colony?

One that would both keep it secret up until modern times but also have the resources to be self-sufficient? (Portions of Australia wilderness or northern Canada seem as likely candidates)

Since this colony is built for military application, they likely would be somewhere near the coasts(but obviously this could cause water issues. However I thought perhaps water could be used as power for the colony's mining needs.)

2. And realistically, where underground would the colony be built?

Obviously water-table and fault lines will play a role. (Though fault lines weren't discovered until 1915 if my memory is correct) I am aware of certain theories about the Earth's crust being "honeycombed". If so, perhaps a network of these large caverns would do. What about vacated magma tubes? Are there other natural structures that would provide a good area from which to house an underground colony?

Additional details:

• The technology in question is early 20th century industrial equipment.
• There is electricity involved.
• The colony does have access to the outside world, but is designed to be able to live mostly underground.
• The colony is nearly completely self-sufficient, including their own steel-making capabilities and people trained in weapons manufacturing
• This is a large-scale colony growing to about 20,000 before being abandoned. And obviously larger in the years that followed.

Then :

• The project is highly classified.
• Sufficient funding is backing the project.
• They have over two decades to complete it.

While I research various scientific studies, I am chiefly interested in robotics and astronomy. So, I am not up-to-date concerning much of geology. So please correct me if I have made any errors.

Answer 1

The Dengie Nature reserve

Dengie nature reserve is a 3,105 hectare biological and geological Site of Special Scientific Interest between the estuaries of the Blackwater and Crouch near Bradwell-on-Sea in Essex.[2][3] It is also a National Nature Reserve,[4] a Special Protection Area,[5] a Nature Conservation Review site,[2] a Geological Conservation Review site[6] and a Ramsar site.[7] It is part of the Essex estuaries Special Area of Conservation.[8] An area of 12 hectares is the Bradwell Shell Bank nature reserve, which is managed by the Essex Wildlife Trust.[9]

It consists of large, remote area of tidal mud-flats and salt marshes at the eastern end of the Dengie peninsula . The Chapel of St Peter-on-the-Wall overlooks some of the site.

Nature reserve. Sure it is. Your colony was originally sited here chiefly because of convenient access to London. The morlock-like residents of the colony do not know that they are still on England.

Answer 2

You need a location with a temperate climate and a low indigenous population.

Western or eastern seaboard Canada has potential, but inquisitive Americans are very near. The Americans could easily ascertain what would be occurring there and its precise location. The number of indigenous people could be problematic. They could easily act as spies for whomever. Relocating such large numbers of people would be difficult and problematic.

South Africa has the climate, but too many indigenous people.

Concerning Australia, on the mainland, the most likely locations would be the south west corner, between Perth and Albany; the east coast from Cairns, in the north down to Portland, in western Victoria, in the south. Also, anywhere on the island of Tasmania (during its convict period, it was known as van Diemen's Land). Tasmania is the large valentine shaped island in the south east of Australia. Most of these locations in Australia offer potential to use hydroelectric power, particularly north west and north eastern Tasmania and the mid eastern coast of the mainland. Ignore the rest of Australia, it's too inhospitable.

Most of New Zealand would also be a potential location. If you want easy access to the oceans but some seclusion then the Fjordland region in the south west could be a good location.

The trouble with anywhere in Australia or New Zealand is these would be remote outpost locations far from the main sailing routes, but for British naval activity between India and China, Japan or eastern Russia then maybe they could be useful.

Answer 3

British Guyana

Britain had one colony in mainland South America. It was a bit of a backwater and in real history, operated by absentee slave owners. Its main exports were bauxite and gold. Just keep out the pesky Venezuelans. The Spanish got kicked out in 1811, so the only developed nation likely to come snooping is the USA, and even that is less likely than in Canada, plus bodies can be easily hidden in the jungle.