The Neonian Baptistery

**Because the city was built on marsh and is still subsiding — and the difference runs to several metres.** ✅ **The origin**: Ravenna was built **on wet, alluvial ground** among marshes and a river delta — **which is what made it militarily secure**, and the reason it was chosen as capital. ⚠ **But alluvial soil compresses**: **wet clay and sand consolidate slowly under the weight of buildings** over centuries — **so the buildings sink**. ✅ **Two things compound it**: **groundwater extraction** in modern times sharply accelerated the subsidence, **and relative sea-level rise**. ✅ **The visible result**: **the Neonian Baptistery's floor is some three metres below the street**, **and many of Ravenna's churches are entered by going down** rather than up — **the opposite of the usual**. ⚠ **In some buildings water marks show** on the lower walls. ✅ **The most striking example: the church of San Francesco** — **its crypt is permanently flooded**, **with goldfish swimming above a 10th-century mosaic floor**; **seen through an opening with coin-operated lighting**, ✅ **free, and worth the detour**. ⚠ **The subsidence is a genuine conservation problem** and engineering effort goes into slowing it.