diff --git a/source/core/maptypes.h b/source/core/maptypes.h
index dcb946a68..055ef076e 100644
--- a/source/core/maptypes.h
+++ b/source/core/maptypes.h
@@ -100,6 +100,7 @@ enum ESectorExBits
 	SECTOREX_CLOUDSCROLL			= 1,
 	SECTOREX_DRAGGED				= 2,
 	SECTOREX_DONTCLIP				= 4,
+	SECTOREX_SEEN					= 8,
 };
 
 // Flags for retriangulation
diff --git a/source/core/rendering/scene/hw_flats.cpp b/source/core/rendering/scene/hw_flats.cpp
index 80e75ccfa..7490ac5bb 100644
--- a/source/core/rendering/scene/hw_flats.cpp
+++ b/source/core/rendering/scene/hw_flats.cpp
@@ -319,6 +319,7 @@ void HWFlat::ProcessSector(HWDrawInfo *di, sectortype * frontsector, int section
 	PlanesAtPoint(frontsector, vp.Pos.X, -vp.Pos.Y, &ceilz, &florz);
 
 	visibility = sectorVisibility(frontsector);
+	frontsector->exflags |= SECTOREX_SEEN;
 	sec = frontsector;
 	section = section_;
 	Sprite = nullptr;
diff --git a/source/games/duke/src/actors_lava.cpp b/source/games/duke/src/actors_lava.cpp
index d148962dd..97bd8f8c8 100644
--- a/source/games/duke/src/actors_lava.cpp
+++ b/source/games/duke/src/actors_lava.cpp
@@ -479,8 +479,18 @@ void thunder(void)
 	}
 	if (!winderflash && isRR())
 	{
-		auto tex = tileGetTexture(RTILE_CURTAINS);	// this cannot be easily generalized. :(
-		if (tex->isSeen(true))
+		bool seen = false;
+		for (i = 0; i < lightnincnt; i++)
+		{
+			auto sectp = lightninsector[i];
+			if (sectp->exflags & SECTOREX_SEEN)
+			{
+				seen = true;
+				sectp->exflags &= ~SECTOREX_SEEN;
+			}
+		}
+
+		if (seen)
 		{
 			if (krand() > 65000)
 			{