Underground

In the final episode Piers and Caroline's journey starts on the Greek island of Anti Paros, to visit a nine-bedroom house that is hidden beneath the landscape and yet still achieves stunning sea views. As they discover, the green roof allows the underground spaces to be invisible so only the pool terrace and white-washed walls are on view.

Their next stop takes them to the lush valleys of the Swiss Alps, where the owner architect made his four-bedroom house so invisible it has to be accessed via a tunnel from the traditional agricultural barn that was already on the site. Piers and Caroline emerge into the main house, its concave façade revealing panoramic views of the surrounding mountains.

Next it's over to New Zealand's South Island, to a house that was built underground to soften the impact on the indigenous landscape while at the same time being heavily engineered to withstand the threat of earthquakes. Dynamite was used to excavate 5,000 cubic metres of earth and rock to create the three-bedroom main house and annex, constructed from concrete and cloaked with an engineered wing-shaped roof.

Lastly, Caroline and Piers cycle to visit a very different underground house nestled in a nature reserve in the outskirts of Amsterdam, Holland. The four-bedroom family house was created by deep excavation and then by stacking the bedrooms and bathrooms across three floors on the north side, allowing for a huge light-filled open-plan living space facing south.