Thanks to an intrepid group of urban explorers, some of the most magnificent hidden engineering triumphs that lie, hidden, beneath the streets of the worlds cities are being recorded and posted on underground (no pun intended) websites.Here are some of our favorite sewers and drains, from Paris tourist attraction sewers to Londons Escher-like, arched, Victorian Gothic drains, to still working ancient Roman systems and the infamous giant storm drains beneath Tokyo. Vote for your faves.

In this kind of space, no one can hear you scream. Anechoic chambers use spiked walls to eliminate echoes, the end result might literally sound dull but the visual effect can be stunning, such as at the enormous anechoic hangar. Vote for your faves.

Climbing walls are both functionally and aesthetically fascinating. They often have beautiful abstract shapes reminiscent of Kurt Schwitters Merzbau or are just plain intricate and impressive.Included in our collection here are interactive musical climbing walls, enormous artificial ice towers, surreal climbing forms and a huge climbing wall inside a disused Texan grain silo. Vote for your faves.

The interior and exterior of wind tunnels have unusual design requirements that often make them accidental architectural masterpieces.Wind tunnels range from the miniature wooden box that the Wright brothers used, to the gigantic full scale tunnel at NASA Ames in Silicon Valley which drains the power supply for the entire Bay Area, and tests actual planes and space craft. Air speeds within them can reach tens of times the speed of sound, requiring super heated air.All in all, they are definitely object to go ooh about.

Singapore Airlines has this week banished the mile high club, with the introduction of on board double beds. The rest of the interior, however is fairly bland. Here is a list of some of the best aircraft interiors.