We asked our readers to send in their best pictures on the theme of "tunnels". Here is a selection of the photographs we received from around the world.
Paul Kalas
Paul Kalas: “Swimming one-handed with the selfie-stick in my left hand through a 30m (100ft) long rock tunnel at Spilies Beach on the north coast of Crete, Greece.”
Tamás Palotás
Tamás Palotás: “Castle Tunnel of Budapest, Hungary, designed by the Scotsman Adam Clark, looks in the evening with its vibrant lights like the stomach of a cachalot, which has been swallowing urban traffic since opening in 1857.”
Jacqui Dimmock
Jacqui Dimmock: “I took a quick break whilst cycling to take this photo inside one of tunnels along the Monsal Trail in the Peak District. After cycling a fair few yards in, I was still seeking the light at the end of it.”
Peter Carter
Peter Carter: “The train now ing through the tunnel at Grosmont is the 14:40 to Pickering. The tunnel is part of the North Yorkshire Moors Railway line, and many people like to see the steam trains ing through, as it brings back memories of past travel, with the smell and soot particles of the burning coal, the hissing of the escaping steam and the wonderful sound of the steam whistle.”
Tony Dowell
Tony Dowell: “The view from a natural tunnel in the walls of the Rocca Maggiore citadel in Assisi, Italy. The morning mist framing the peace and tranquillity of the town of St Francis.”
Victor Tregubov: “A tunnel connecting different terminals at Zurich Airport.”
Phil Couvrette
Phil Couvrette: “Canada's oldest railway tunnel no longer sees any trains in its 500m (1,640ft) stretch in downtown Brockville, but it's become a visitor's favourite, its limestone rock interior now a colourful canvas thanks to soft LED lights glowing to instrumental music.”
Ali Lawrence
Ali Lawrence: “Water flowing through tunnels at Bytheway Field in Wimborne, highlighting the different textures between the metal tunnels, the flowing water and the moss and leaves. “
Louise Turvey
Louise Turvey: “Two strangers iring the mountain view at Aiguille du Midi in the French Alps.”
Mathias Vejersle
Mathias Vejersle: “The driverless Copenhagen Metro on long exposure, captured here within the first few weeks of operation back in 2003.”
Neil Marsden
Neil Marsden: “This filter effect seems to create tunnel through the trees and bridges in the parks around Merthyr Tydfil.”
Alice Lenton
Alice Lenton: “Crouching through the entrance of a glacial Icelandic ice cave, I looked back up the glassy tunnel and was met with an otherworldly sight.”
Ginnie Kelly
Ginnie Kelly: “My daughter, Olive, climbing up a tunnel in Battersea Park Children’s Zoo. I like the silhouette of her within the circles of light created by the corrugated metal.”
Alan Mackay
Alan Mackay: “It's OK lads we've almost made it to the fence. Let’s hope for a moonless night.”
The next theme is "ripples" and the deadline for entries is 28 May 2024.
The pictures will be published later that week and you will be able to find them, along with other galleries, on the In Pictures section of the BBC News website.