You are at a place that used to be the private hunting grounds of medieval kings. This 95 hectare park, called Stromovka, was founded around 1268 by King Přemysl Otakar II. For centuries, only royalty hunted deer and wild game here.
Then Rudolf II showed up.
The eccentric Holy Roman Emperor wanted bigger fish ponds in his park. Problem was, the existing ponds could not hold enough water. So in 1582, he ordered miners from the Kutná Hora silver mines to dig a tunnel from the Vltava River straight through solid rock beneath what is now Letná Park.
No dynamite. Just hammers and chisels.
The miners worked from both ends simultaneously through 5 shafts, some 39 meters deep. They carved an oval tunnel 1,102 meters long, barely wide enough for a person to squeeze through in places. The whole project cost 66,299 silver groschens and took 11 years to complete. When the two teams met in the middle, they were slightly off, which is why one section is wider than the rest.
The tunnel still works. Right now, gravity is pulling Vltava water through that Renaissance era engineering marvel into the ponds you see around you. The entrance sits nearby, marked with Rudolf's crown and the letter R carved into stone in 1583.
The park opened to the public in 1804. The name Stromovka, meaning place of trees, came later when Germans called it Baumgarten.