Ever since the completion of the Great Western Railway, in the 1840s, intrigue has swirled around the Box Tunnel, a long, steep bypass near Bath, England. Unlike many tunnel rumors, these were sweet rather than sordid. The question was this: Did the railway’s creator, Isambard Kingdom Brunel, really have the tunnel carved in such a way that when the sun rose on his birthday—April 9th—it would be flooded with light?

On April 9th, 2017, the railway’s engineers decided to test the rumor once and for all. They weren’t disappointed. “When you look from the east portal, the cutting provides a lovely V-shape,” communications manager Paul Gentleman told the Guardian. “The sun rose from the left and was shining directly down the tunnel. We couldn’t see how far.”

The tunnel, which stretches nearly two miles between Bath and Chippenham, was completed in 1837. The idea that Brunel had included this Easter egg began circulating soon after, in an article in the Bristol Mirror in 1842. Since then, historians and mathematicians have attempted to determine whether the trick is atmospherically and geometrically possible, and have come to various conclusions. In 2017, the line was closed for construction, giving the engineers a good opportunity to see for themselves.

Another fascinating chapter in the box tunnel’s history was its use for defense purposes. The tunnel was taken up by munitions storage during World War II, and during the Cold War portions of it were used as an intelligence headquarters. In the 1980s, a museum was briefly opened up inside the tunnel showcasing this history, but today there is little to no evidence of it.